Tag Archives: flutter dart

Flutter & Dart every() Method Example

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Flutter & Dart: every() method example

A few examples of using the every() method ( Iterable class) in Dart (and Flutter) . The purpose of this method is to check whether each element of a given iteration satisfies one or more conditions (using a test function):

bool every(
   bool test(
     E element
   )
)

1. Check that each number in the list is divisible by 3 :

void main() {
  var myList = [0, 3, 6, 9, 18, 21, 81, 120];
  bool result1 = myList.every((element){
    if(element %3 ==0){
      return true;
    } else {
      return false; 
    }
  }); 
  
  // expectation: true
  print(result1); 
}

output:

true

2. Check that each name in the list contains the “m” character :

void main() {
  var names = ['Obama', 'Trump', 'Biden', 'Pompeo']; 
  bool hasM = names.every((element) {
     if(element.contains('m')) return true;
     return false;
  });
  
  print(hasM);
}

output:

false

You can find more details about every() method in the official documentation.

Flutter & Dart Enumeration Example

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Flutter & Dart: Enumeration Example

This article will show you how to use enums (also known as enums or enum types) in Dart and Flutter.

Overview

An enumeration in Dart is a set of symbolic names (members) bound to a unique constant value. In an enumeration, members can be compared by identity, and the enumeration itself can be iterated over.

An enumeration can be declared using the enum keyword:

enum Aniaml {dog, cat, chicken, dragon}

Each value of the enumeration has an index. The index of the first value is 0.

You can retrieve all values ​​of an enumeration using the values ​​constant:

print(Aniaml.values);
// [Aniaml.dog, Aniaml.cat, Aniaml.chicken, Aniaml.dragon]

a complete example

coding:

// Declare enum
enum Gender {
  Male,
  Female
}
​
// Make a class
class Person {
  final String name;
  final int age;
  final Gender gender;
  
  Person(this.name, this.age, this.gender); 
}
​
// Create an instance
final personA = Person("John Doe", 40, Gender.Male); 
​
void main(){
  if(personA.gender == Gender.Male){
    print("Hello gentleman!");
  } else {
    print("Hello lady!");
  }
}

output:

Hello gentleman!

Flutter & Dart: Check if a date is between two other dates

To check if a date is between two other dates in Flutter and Dart, you can use the isBefore( ) and isAfter() methods of the DateTime class .

example

import 'package:flutter/foundation.dart';
​
DateTime dateA = DateTime(1900, 9, 14);
DateTime dateB = DateTime(2000, 10, 15);
​
DateTime dateC = DateTime(1984, 12, 20);
DateTime dateD = DateTime.now();
​
void main() {
  if (dateA.isBefore(dateC) && dateB.isAfter(dateC)) {
    debugPrint("dateC is between dateA and dateB");
  } else {
    debugPrint("dateC isn't between dateA and dateC");
  }
​
  if (dateA.isBefore(dateD) && dateB.isAfter(dateD)) {
    debugPrint("dateD is between dateA and dateB");
  } else {
    debugPrint("dateD isn't between dateA and dateC");
  }
}

output:

dateC is between dateA and dateB
dateD isn't between dateA and dateC

You can find more information about DateTime class in the official documentation.

Flutter & Dart Regular Expression Examples

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Flutter & Dart: Regular Expression Examples

Example 1: Email Verification

coding:

void main() {
  RegExp exp = new RegExp(
      r"^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9-]+)*$",
      caseSensitive: false);
​
  // Try it
  String email1 = "[email protected]";
  String email2 = "test@gmail";
  String email3 = "test#gmail.com";
​
  if (exp.hasMatch(email1)) {
    print("Email1 OK");
  }
​
  if (exp.hasMatch(email2)) {
    print("Email2 OK");
  }
​
  if (exp.hasMatch(email3)) {
    print("Email3 OK");
  }
}

output:

Email1 OK
Email2 OK

Example 2: IP address verification

coding:

void main() {
  RegExp ipExp = new RegExp(r"^(?!0)(?!.*\.$)((1?\d?\d|25[0-5]|2[0-4]\d)(\.|$)){4}$", caseSensitive: false, multiLine: false);
  
  // Try it
  // Expectation: true
  if(ipExp.hasMatch('192.168.1.2')){
    print('192.168.1.2 is valid'); 
  }
  
  // Expectation: false
  if(ipExp.hasMatch('192.168.111111.55555')){
    print('192.168.111111.55555 is valid');
  }
}

output:

192.168.1.2 is valid

Example 3: URL Validation

coding:

void main() {
  RegExp urlExp = RegExp(r"(http|ftp|https)://[\w-]+(\.[\w-]+)+([\w.,@?^=%&:/~+#-]*[\w@?^=%&/~+#-])?");
  
  String url1 = "https://www.kindacode.com/cat/mobile/flutter/"; // valid
  
  String url2 = "https://kindacode/cat/mobile/flutter/"; // invalid
  
  if(urlExp.hasMatch(url1)) {
    print('Url1 looks good!');
  }
  
  if(urlExp.hasMatch(url2)) {
    print("Url2 looks good!");
  }
}

output:

Url1 looks good!

Example 4: Domain Validation

void main() {
  RegExp  domainExp = RegExp(r"^[a-z0-9]+([\-\.]{1}[a-z0-9]+)*\.[a-z]{2,6}$");
  
  String domain1 = "www.kindacode.com"; // valid
  String domain2 = "www.amazon.co.uk"; // valid
  String domain3 = "hello world!"; // invalid
  
  if(domainExp.hasMatch(domain1)){
    print('domain1 is valid');
  }
  
  if(domainExp.hasMatch(domain2)){
    print('domain2 is valid');
  }
  
  if(domainExp.hasMatch(domain3)){
    print('domain3 is valid');
  }
}

output:

domain1 is valid
domain2 is valid

Flutter & Dart: Conditionally remove elements from a list

To remove elements from a list based on one or more conditions, you can use the built-in removeWhere() method.

example

Suppose we have a list of products and the task is to remove all products with a price higher than 100:

// Define how a product looks like
class Product {
  final double id;
  final String name;
  final double price;
  Product(this.id, this.name, this.price);
}

void main() {
  // Generate the product list
  List<Product> products = [
    Product(1, "Product A", 50),
    Product(2, "Product B", 101),
    Product(3, "Product C", 200),
    Product(4, "Product D", 90),
    Product(5, "Product E", 400),
    Product(6, "Product F", 777)
  ];

  // remove products whose prices are more than 100
  products.removeWhere((product) => product.price > 100);

  // Print the result
  print("Product(s) whose prices are less than 100:");
  for (var product in products) {
    print(product.name);
  }
}

output:

Product(s) whose prices are less than 100:
Product A
Product D

Reference : removeWhere method (dart.dev).

Summarize

We’ve gone through some examples of using regular expressions in Dart that can be very useful in many common use cases. If you want to learn more about Dart and Flutter, you can follow me.

How to Use Annotations in Flutter & Dart

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How to use annotations in Flutter & Dart

This is a concise article on Flutter and Dart annotations.

Single Line Comments (Format Comments)

Just put two slash symbols at the beginning of the line you want to comment out.

// This is a comment
print('abc');

Multi-line comments (block comments)

syntax:

/* ... */

This method is often used to comment out a piece of code like this:

/*
class HomeScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Kindacode.com'),
      ),
      body: Center(),
    );
  }
}
*/

Documentation comments

Using DOC comments allows the dartdoc library to automatically generate documentation for your code based on the content of your comments.

example:

/// Deletes the file at [path].
/// Throws an [IOError] if the file could not be found.

Use hotkeys in Visual Studio Code

If you’re using Visual Studio Code, you can use keyboard shortcuts to comment out a line or block of code. Just select the line you want to comment with your mouse and press the following key combination:

  • If you are using Windows, press Ctrl + K then Ctrl + C

  • If you’re on a Mac, press Command + K then Command + C

To uncomment a block of code, select it with the mouse, then use the following key combinations:

  • Ctrl + K then Ctrl + U if you are on Windows

  • If you’re on a Mac, Command + K then Command + U

You can also switch by pressing Ctrl + / (Windows), Command + / (Mac).

How to encode/decode JSON in Flutter

shorthand

This article will show you how to encode/decode JSON in Flutter. The steps you need to follow are:

1. Import the dart:convert library:

import 'dart:convert';

2. Use:

  • json.encode() or jsonEncode() for encoding.

  • json.decode() or jsonDecode() for decoding.

example

Example 1: JSON encoding

import 'dart:convert';
​
void main() {
  final products = [
    {'id': 1, 'name': 'Product #1'},
    {'id': 2, 'name': 'Product #2'}
  ];
​
  print(json.encode(products));
}

output:

[{"id":1,"name":"Product #1"},{"id":2,"name":"Product #2"}]

Example 2: JSON decoding

import 'dart:convert';
import 'package:flutter/foundation.dart';
​
void main() {
  const String responseData =
      '[{"id":1,"name":"Product #1"},{"id":2,"name":"Product #2"}]';
​
  final products = json.decode(responseData);
​
  if (kDebugMode) {
    // Print the type of "products"
    print(products.runtimeType);
​
    // Print the name of the second product in the list
    print(products[1]['name']);
  }
}

output:

List<dynamic>
Product #2

Hope this helps you.

Mac installation dart error curl: (35) libresssl SSL_ connect: SSL_ ERROR_ SYSCALL in connection to storage.googleapis.co

!! Close test effectively
MAC installation dart error curl: (35) LibreSSL SSL_connect: SSL_ERROR_SYSCALL in connection to storage.googleapis.com: 443
Install the brew:
Console operation:
1. Install the brew

/bin/zsh -c "$(curl -fsSL https://gitee.com/cunkai/HomebrewCN/raw/master/Homebrew.sh)"

2. Install the dart – lang/dart

brew tap dart-lang/dart

3. Install the dart

brew install dart

If this operation succeeds, it ends
4. Report an error

➜  canvasVueTemp git:(master) ✗ brew install dart      
==> Installing dart from dart-lang/dart
==> Downloading https://storage.googleapis.com/dart-archive/channels/stable/release/2.7.2/sdk/dartsdk-macos-x64-release.zip

curl: (35) LibreSSL SSL_connect: SSL_ERROR_SYSCALL in connection to storage.googleapis.com:443 
Error: An exception occurred within a child process:
  DownloadError: Failed to download resource "dart"
Download failed: https://storage.googleapis.com/dart-archive/channels/stable/release/2.7.2/sdk/dartsdk-macos-x64-release.zip

Solutions:
Resolved Homebrew installation software download failure
When we use Homebrew to install software, the software package download fails due to some special reasons. This is still very common and we can’t change the environment, but we can take advantage of The Homebrew caching feature by manually downloading software in advance.

$ brew install dart
==> Installing dart from dart-lang/dart
==> Downloading https://storage.googleapis.com/dart-archive/channels/stable/release/2.4.1/sdk/dartsdk-macos-x64-release.zip

curl: (56) LibreSSL SSL_read: SSL_ERROR_SYSCALL, errno 54
Error: An exception occurred within a child process:
  DownloadError: Failed to download resource "dart"
Download failed: https://storage.googleapis.com/dart-archive/channels/stable/release/2.4.1/sdk/dartsdk-macos-x64-release.zip

The corresponding software package cannot be successfully downloaded, but Homebrew will tell the software download address:

Download failed: https://storage.googleapis.com/dart-archive/channels/stable/release/2.4.1/sdk/dartsdk-macos-x64-release.zip

So, we can download the software manually. Then we get the cache directory:

$ brew --cache
/Users/shockerli/Library/Caches/Homebrew

Copy the package you just downloaded to this directory:

$ cp ~/Downloads/dartsdk-macos-x64-release.zip /Users/shockerli/Library/Caches/Homebrew/

We execute the installation command again, no accident, so congratulations, successfully solved the problem.
But accidents happen, and unfortunately you, like me, find that you still give the wrong report:

$ brew install dart
==> Installing dart from dart-lang/dart
==> Downloading https://storage.googleapis.com/dart-archive/channels/stable/release/2.4.1/sdk/dartsdk-macos

curl: (56) LibreSSL SSL_read: SSL_ERROR_SYSCALL, errno 54
Error: An exception occurred within a child process:
  DownloadError: Failed to download resource "dart"
Download failed: https://storage.googleapis.com/dart-archive/channels/stable/release/2.4.1/sdk/dartsdk-macos-x64-release.zip

So what’s the solution?We add a -v to the command to print the detailed log look:

brew install dart -v
==> Installing dart from dart-lang/dart
/usr/bin/sandbox-exec -f /private/tmp/homebrew20190810-60798-mlb2s.sb nice /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby -W0 -I /usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/simplecov-cobertura-1.3.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ruby-macho-2.2.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rubocop-rspec-1.35.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rubocop-performance-1.4.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rubocop-0.74.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/unicode-display_width-1.6.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ruby-progressbar-1.10.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-wait-0.0.9/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-retry-0.6.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-its-1.3.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-3.8.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-mocks-3.8.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-expectations-3.8.4/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-core-3.8.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-support-3.8.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ronn-0.7.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rdiscount-2.2.0.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/rdiscount-2.2.0.1:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rainbow-3.0.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/plist-3.5.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/parser-2.6.3.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/parallel_tests-2.29.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/parallel-1.17.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mustache-1.1.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mechanize-2.7.6/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/webrobots-0.1.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ntlm-http-0.1.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/nokogiri-1.10.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/nokogiri-1.10.3:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mini_portile2-2.4.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/net-http-persistent-3.1.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/net-http-digest_auth-1.4.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mime-types-3.2.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mime-types-data-3.2019.0331/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/jaro_winkler-1.5.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/jaro_winkler-1.5.3:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/http-cookie-1.0.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/hpricot-0.8.6/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/hpricot-0.8.6:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/domain_name-0.5.20190701/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/unf-0.1.4/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/unf_ext-0.0.7.6/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/unf_ext-0.0.7.6:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/diff-lcs-1.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/coveralls-0.8.23/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/thor-0.20.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/term-ansicolor-1.7.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/tins-1.21.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/simplecov-0.16.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/simplecov-html-0.10.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/docile-1.3.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/json-2.2.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/json-2.2.0:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/connection_pool-2.2.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/backports-3.15.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ast-2.4.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/activesupport-5.2.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/tzinfo-1.2.5/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/thread_safe-0.3.6/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/minitest-5.11.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/i18n-1.6.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/concurrent-ruby-1.1.5/lib:/Library/Ruby/Site/2.3.0:/Library/Ruby/Site/2.3.0/x86_64-darwin18:/Library/Ruby/Site/2.3.0/universal-darwin18:/Library/Ruby/Site:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/vendor_ruby/2.3.0:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/vendor_ruby/2.3.0/x86_64-darwin18:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/vendor_ruby/2.3.0/universal-darwin18:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/vendor_ruby:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/x86_64-darwin18:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/universal-darwin18:/usr/local/Homebrew/Library/Homebrew -- /usr/local/Homebrew/Library/Homebrew/build.rb /usr/local/Homebrew/Library/Taps/dart-lang/homebrew-dart/dart.rb --verbose

==> Downloading https://storage.googleapis.com/dart-archive/channels/stable/release/2.4.1/sdk/dartsdk-macos-x64-release.zip
/usr/bin/curl -q --show-error --user-agent Homebrew/2.1.9-21-g625a780\ \(Macintosh\;\ Intel\ Mac\ OS\ X\ 10.14.2\)\ curl/7.54.0 --fail --location --remote-time --continue-at 0 --output /Users/shockerli/Library/Caches/Homebrew/downloads/4d2412a5d84521393e0e1ecdce0662569e13c2c47762093a760939fa9dd4a917--dartsdk-macos-x64-release.zip.incomplete https://storage.googleapis.com/dart-archive/channels/stable/release/2.4.1/sdk/dartsdk-macos-x64-release.zip
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:--  0:00:43 --:--:--     0
curl: (56) LibreSSL SSL_read: SSL_ERROR_SYSCALL, errno 60
Error: An exception occurred within a child process:
  DownloadError: Failed to download resource "dart"
Download failed: https://storage.googleapis.com/dart-archive/channels/stable/release/2.4.1/sdk/dartsdk-macos-x64-release.zip

Notice this message:

/usr/bin/curl -q --show-error --user-agent Homebrew/2.1.9-21-g625a780\ \(Macintosh\;\ Intel\ Mac\ OS\ X\ 10.14.2\)\ curl/7.54.0 --fail --location --remote-time --continue-at 0 --output /Users/shockerli/Library/Caches/Homebrew/downloads/4d2412a5d84521393e0e1ecdce0662569e13c2c47762093a760939fa9dd4a917--dartsdk-macos-x64-release.zip.incomplete https://storage.googleapis.com/dart-archive/channels/stable/release/2.4.1/sdk/dartsdk-macos-x64-release.zip

We can see that the cache address of Homebrew download Dart is:

/Users/shockerli/Library/Caches/Homebrew/downloads/4d2412a5d84521393e0e1ecdce0662569e13c2c47762093a760939fa9dd4a917--dartsdk-macos-x64-release.zip.incomplete

Incomplete means that the download is not complete, but this is the path to the download file that Homebrew expects.
2. Then we will copy the downloaded package into this path, and remove. Incomplete:

mv ~/Downloads/dartsdk-macos-x64-release.zip /Users/shockerli/Library/Caches/Homebrew/downloads/4d2412a5d84521393e0e1ecdce0662569e13c2c47762093a760939fa9dd4a917--dartsdk-macos-x64-release.zip

At this point, we execute the installation command again:

brew install dart -v
==> Installing dart from dart-lang/dart
/usr/bin/sandbox-exec -f /private/tmp/homebrew20190811-62563-1rnamem.sb nice /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby -W0 -I /usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/simplecov-cobertura-1.3.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ruby-macho-2.2.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rubocop-rspec-1.35.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rubocop-performance-1.4.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rubocop-0.74.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/unicode-display_width-1.6.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ruby-progressbar-1.10.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-wait-0.0.9/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-retry-0.6.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-its-1.3.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-3.8.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-mocks-3.8.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-expectations-3.8.4/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-core-3.8.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-support-3.8.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ronn-0.7.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rdiscount-2.2.0.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/rdiscount-2.2.0.1:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rainbow-3.0.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/plist-3.5.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/parser-2.6.3.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/parallel_tests-2.29.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/parallel-1.17.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mustache-1.1.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mechanize-2.7.6/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/webrobots-0.1.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ntlm-http-0.1.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/nokogiri-1.10.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/nokogiri-1.10.3:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mini_portile2-2.4.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/net-http-persistent-3.1.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/net-http-digest_auth-1.4.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mime-types-3.2.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mime-types-data-3.2019.0331/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/jaro_winkler-1.5.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/jaro_winkler-1.5.3:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/http-cookie-1.0.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/hpricot-0.8.6/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/hpricot-0.8.6:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/domain_name-0.5.20190701/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/unf-0.1.4/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/unf_ext-0.0.7.6/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/unf_ext-0.0.7.6:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/diff-lcs-1.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/coveralls-0.8.23/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/thor-0.20.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/term-ansicolor-1.7.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/tins-1.21.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/simplecov-0.16.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/simplecov-html-0.10.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/docile-1.3.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/json-2.2.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/json-2.2.0:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/connection_pool-2.2.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/backports-3.15.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ast-2.4.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/activesupport-5.2.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/tzinfo-1.2.5/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/thread_safe-0.3.6/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/minitest-5.11.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/i18n-1.6.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/concurrent-ruby-1.1.5/lib:/Library/Ruby/Site/2.3.0:/Library/Ruby/Site/2.3.0/x86_64-darwin18:/Library/Ruby/Site/2.3.0/universal-darwin18:/Library/Ruby/Site:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/vendor_ruby/2.3.0:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/vendor_ruby/2.3.0/x86_64-darwin18:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/vendor_ruby/2.3.0/universal-darwin18:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/vendor_ruby:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/x86_64-darwin18:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/universal-darwin18:/usr/local/Homebrew/Library/Homebrew -- /usr/local/Homebrew/Library/Homebrew/build.rb /usr/local/Homebrew/Library/Taps/dart-lang/homebrew-dart/dart.rb --verbose
==> Downloading https://storage.googleapis.com/dart-archive/channels/stable/release/2.4.1/sdk/dartsdk-macos-x64-release.zip


Already downloaded: /Users/shockerli/Library/Caches/Homebrew/downloads/4d2412a5d84521393e0e1ecdce0662569e13c2c47762093a760939fa9dd4a917--dartsdk-macos-x64-release.zip
==> Verifying 4d2412a5d84521393e0e1ecdce0662569e13c2c47762093a760939fa9dd4a917--dartsdk-macos-x64-release.zip checksum
unzip /Users/shockerli/Library/Caches/Homebrew/downloads/4d2412a5d84521393e0e1ecdce0662569e13c2c47762093a760939fa9dd4a917--dartsdk-macos-x64-release.zip -d /private/tmp/d20190811-62564-1me3qph
cp -pR /private/tmp/d20190811-62564-1me3qph/dart-sdk/. /private/tmp/dart-20190811-62564-hu8qrn/dart-sdk
chmod -Rf +w /private/tmp/d20190811-62564-1me3qph
==> Cleaning
==> Finishing up
ln -s ../Cellar/dart/2.4.1/bin/dart dart
ln -s ../Cellar/dart/2.4.1/bin/dart2aot dart2aot
ln -s ../Cellar/dart/2.4.1/bin/dart2js dart2js
ln -s ../Cellar/dart/2.4.1/bin/dartanalyzer dartanalyzer
ln -s ../Cellar/dart/2.4.1/bin/dartaotruntime dartaotruntime
ln -s ../Cellar/dart/2.4.1/bin/dartdevc dartdevc
ln -s ../Cellar/dart/2.4.1/bin/dartdoc dartdoc
ln -s ../Cellar/dart/2.4.1/bin/dartfmt dartfmt
ln -s ../Cellar/dart/2.4.1/bin/pub pub
/usr/bin/sandbox-exec -f /private/tmp/homebrew20190811-62996-w9cwbv.sb nice /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby -W0 -I /usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/simplecov-cobertura-1.3.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ruby-macho-2.2.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rubocop-rspec-1.35.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rubocop-performance-1.4.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rubocop-0.74.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/unicode-display_width-1.6.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ruby-progressbar-1.10.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-wait-0.0.9/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-retry-0.6.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-its-1.3.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-3.8.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-mocks-3.8.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-expectations-3.8.4/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-core-3.8.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rspec-support-3.8.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ronn-0.7.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rdiscount-2.2.0.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/rdiscount-2.2.0.1:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/rainbow-3.0.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/plist-3.5.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/parser-2.6.3.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/parallel_tests-2.29.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/parallel-1.17.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mustache-1.1.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mechanize-2.7.6/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/webrobots-0.1.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ntlm-http-0.1.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/nokogiri-1.10.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/nokogiri-1.10.3:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mini_portile2-2.4.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/net-http-persistent-3.1.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/net-http-digest_auth-1.4.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mime-types-3.2.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/mime-types-data-3.2019.0331/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/jaro_winkler-1.5.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/jaro_winkler-1.5.3:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/http-cookie-1.0.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/hpricot-0.8.6/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/hpricot-0.8.6:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/domain_name-0.5.20190701/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/unf-0.1.4/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/unf_ext-0.0.7.6/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/unf_ext-0.0.7.6:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/diff-lcs-1.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/coveralls-0.8.23/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/thor-0.20.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/term-ansicolor-1.7.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/tins-1.21.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/simplecov-0.16.1/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/simplecov-html-0.10.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/docile-1.3.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/json-2.2.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/extensions/universal-darwin-18/2.3.0/json-2.2.0:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/connection_pool-2.2.2/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/backports-3.15.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/ast-2.4.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/activesupport-5.2.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/tzinfo-1.2.5/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/thread_safe-0.3.6/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/minitest-5.11.3/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/i18n-1.6.0/lib:/usr/local/Homebrew/Library/Homebrew/vendor/bundle/bundler/../ruby/2.3.0/gems/concurrent-ruby-1.1.5/lib:/Library/Ruby/Site/2.3.0:/Library/Ruby/Site/2.3.0/x86_64-darwin18:/Library/Ruby/Site/2.3.0/universal-darwin18:/Library/Ruby/Site:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/vendor_ruby/2.3.0:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/vendor_ruby/2.3.0/x86_64-darwin18:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/vendor_ruby/2.3.0/universal-darwin18:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/vendor_ruby:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/x86_64-darwin18:/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/universal-darwin18:/usr/local/Homebrew/Library/Homebrew -- /usr/local/Homebrew/Library/Homebrew/postinstall.rb /usr/local/Homebrew/Library/Taps/dart-lang/homebrew-dart/dart.rb -v
==> Caveats
Please note the path to the Dart SDK:
  /usr/local/opt/dart/libexec
==> Summary
🍺  /usr/local/Cellar/dart/2.4.1: 387 files, 344.4MB, built in 3 minutes 11 seconds

Homebrew successfully used to cache and install 😆 ~ congratulations to you
 
Address:
https://segmentfault.com/a/1190000020125374
https://shockerli.net/post/homebrew-install-download-error/
(Since SF has been officially closed these two days, in order to avoid other people’s encountering this problem and finding no solution, we make a backup here)