Web-specific golden comparisons are no longer supported

The flutter_test package and flutter tool will no longer use the webGoldenComparator top-level variable, and instead use the original goldenFileComparator top-level variable (like the non-web platforms).

For users of flutter_test, these changes will be made automatically.

Originally, WebGoldenComparator was added for the HTML-backend of Flutter web, as it was not possible to create an encoded PNG (byte buffer), and a new API was needed. As the HTML backend is being deprecated and removed, this separate API is no longer necessary.

For most users, no changes are required (other than migrating off the HTML backend, which is not covered here), the flutter tool will automatically configure goldenFileComparator and use it (when using a non-HTML web backend).

For users that implement a custom WebGoldenComparator, you will migrate the implemenation to GoldenFileComparator. Fortunately the Canvas Kit and SkWasm backends already required similar methods (compareButes and updateBytes).

For example:

// Before
class MyWebGoldenComparator extends WebGoldenComparator {
  @override
  Future<bool> compare(double width, double height, Uri golden) {
    // will be removed in the migration
  }

  @override
  Future<bool> update(double width, double height, Uri golden) {
    // will be removed in the migration
  }

  @override
  Future<bool> compareBytes(Uint8List bytes, Uri golden) {
    // will be renamed "compare"
  }

  @override
  Future<bool> updateBytes(Uint8List bytes, Uri golden) {
    // will be renamed "update" and the parameter orders swapped
  }
}

// After
class MyGenericGoldenComparator extends GoldenFileComparator {
  @override
  Future<bool> compare(Uint8List bytes, Uri golden) {
    // used to be "compareBytes"
  }

  @override
  Future<bool> update(Uri golden, Uint8List bytes) {
    // used to be "updateBytes"
  }
}

Landed in version: 3.29.0-0.0.pre
In stable release: 3.29

Relevant Issues:

• Issue 145954, where the HTML renderer was deprecated.
• Issue 160261, where it was proposed to consolidate GoldenFileComparator and WebGoldenComparator.

Relevant PRs:

• PR 161196, where WebGoldenComparator was deprecated and the flutter CLI started using goldenFileComparator.

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