Reland the Dart plugin registry

[blasten]

Relands #74469 and incorporates #77959 from @cyanglaz + additional changes in 829756e, 28f74e4.

The last two commits handle this situation:

1. It doesn't generate generated_main.dart if there isn't need for it. That is, no plugin uses dartPluginClass in the pubspec.

2. generated_main.dart is deleted if there isn't any platform resolution. This is the case if you delete a plugin from pubspec.yaml and then hot reload/restart.

3. Use the new target in all cases.

Fixes #52267

[zanderso]

Does this imply that there should be more validation of this section in the _validate method below? Or would we not even get this far if the dependencies section is broken?

[blasten]

Yup. The YAML parser would fail prior to reaching this section. Also,dependencies should always be present since the flutter sdk section is always present in dependencies.

[jonahwilliams]

Dependencies is not required to be present, but the code you added does cover if it is missing. Consider adding a unit test for this:

  testWithoutContext('FlutterManifest can parse empty dependencies', () async {
    const String manifest = '''
name: test
''';
    final BufferLogger logger = BufferLogger.test();
    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
      manifest,
      logger: logger,
    );

    expect(flutterManifest, isNotNull);
    expect(flutterManifest!.dependencies, isEmpty);
  });

[zanderso]

Here and below, just asking to confirm that these tests can't be testWithoutContext.

[blasten]

There are a few global dependencies that touches other functions as well. I managed to get these three tests to use testWithoutContext, but in doing so, I broke almost all the plugins test.

[zanderso]

@jonahwilliams ptal.

[jonahwilliams]

LGTM with a nit

[jonahwilliams]

Dependencies is not required to be present, but the code you added does cover if it is missing. Consider adding a unit test for this:

  testWithoutContext('FlutterManifest can parse empty dependencies', () async {
    const String manifest = '''
name: test
''';
    final BufferLogger logger = BufferLogger.test();
    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
      manifest,
      logger: logger,
    );

    expect(flutterManifest, isNotNull);
    expect(flutterManifest!.dependencies, isEmpty);
  });

[tvolkert]

This has been verified to have caused #91841