Create a flutter example application that uses multiple client implementations

[brianquinlan]

• Creates an example flutter application (in the style of the flutter gallery) that demonstrates package:http including:
  • how to do configuration (including web configuration)
  • how to pass Client instances through the app
  • simple usage
• Updates the cronet_http and cupertino_http examples to match the new application, except where this would increase the complexity too much.
• The application was created with flutter create -t app. Some interesting files:
  • README.md
  • pkgs/cronet_http/example/lib/main.dart
  • pkgs/cronet_http/example/lib/main.dart
  • pkgs/flutter_http_example/README.md
  • pkgs/flutter_http_example/lib/*
  • pkgs/flutter_http_example/mono_pkg.yaml
  • pkgs/flutter_http_example/pubspec.yaml
  • pkgs/flutter_http_example/test/widget_test.dart

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

[natebosch]

I did not look at stuff other than the Dart. Consider getting a second review from someone with Flutter experience.

[natebosch]

Is it typical flutter practice to check in these generated files?

[brianquinlan]

It is checked in in the flutter gallery:
https://github.com/flutter/gallery/blob/fa031bfe9d131010e7a56ee5d343f9f85b367d64/ios/Runner/Assets.xcassets/LaunchImage.imageset/README.md?plain=1#L3

[natebosch]

Suggested change

	// Copyright (c) 2022, the Dart project authors. Please see the AUTHORS file
	// Copyright (c) 2023, the Dart project authors. Please see the AUTHORS file

[brianquinlan]

This is copied from pkgs/cupertino_http so I think that this copyright is correct.