Towards https://github.com/flutter/flutter/issues/160261.
This is a PR and proposal all-in-one to deprecate, and remove, pending
the removal of the HTML backend, `WebGoldenComparator` (and related
symbols, `webGoldenComparator` and `DefaultWebGoldenComparator`).
The concept of `WebGoldenComparator` was added because the HTML-based
renderer could not support the `GoldenFileComparator` contract:
7141c2a136/packages/flutter_test/lib/src/_matchers_web.dart (L105-L108)
Once the Skia renderer (and now, SkWasm renderer) were added, it was now
possible to support the same API used by the native engine(s). This PR
conditionally, if _not_ using the HTML renderer, uses
`goldenFileComparator` instead, which re-uses the same code that
previously backed `DefaultWebGoldenFileComparator`.
No _new_ logic has been introduced in this change. This might need an
iteration or two to get right.
Feedback welcome!
f2e59e738c