Reland: Skia gold driver test

[blasten]

Re-land for #49905.

The solution is to add import 'package:flutter_test/src/_goldens_web.dart'; to _scuba_test_config_web.dart.

[goderbauer]

/cc @Piinks

[Piinks]

The solution is to add import 'package:flutter_test/src/_goldens_web.dart'; to _scuba_test_config_web.dart.

Do you mean for the roll?

[chingjun]

I have three main concerns on this.

1. Adding the requirement to a subset of package:flutter_test that part of it can't depend on dart:ui, in a library that has always been used in flutter environment, might add maintenance burden in the future.
2. Conditionally imported files should expose the same interface, and the fact that _goldens_io.dart and _goldens_web.dart expose different interface is why we have the error internally. (Due to how dart analyzer works)
3. Requiring that internal code to import a file from src/, especially one that is usually conditionally imported, also increase the likelihood that something to be broken in the future.

Discussed with @blasten just now. To resolve 2 and 3, we could split goldens.dart into two sets of files. goldens.dart, _golden_{io,web},dart, and web_comparator.dart, _web_comparator_{io,web}.dart. Both sets will export the same interface, and the class declaration of WebGoldenComparator will be moved to web_comparator.dart, so that we have the same interface on both web and VM.

I would also suggest that we think about issue 1 as well. We could, maybe add another entrypoint to make it clearer about the limitation, or move the requirement to somewhere more contained and less depended on, like package:flutter_golden.

[blasten]

@chingjun thanks for the suggestion. Thinking this through, it makes sense to just change the WebGoldenComparator interface, so the methods take dynamic types instead of types defined in dart:ui. I'm going to go with this approach since it's simpler. cc @jonahwilliams who had some concerns.

[blasten]

PTAL

[chingjun]

LGTM

[Piinks]

[Piinks]

I was wondering as I've been working on #49815, is there anything in the environment that indicates this is a driver test? Or what device it is running on?

[blasten]

Not in the environment, unfortunately.

enableFlutterDriverExtension() has an assert WidgetsBinding.instance is _DriverBinding. Maybe we could make _DriverBinding public.

cc @jonahwilliams do you have any suggestions?

[Piinks]

Maybe we can just provide a flag then, or include something in the file name. When we get down to the skia client I would like to specify in the parameters that this is a device test and/or the device it is running on, but it may not be necessary.

https://github.com/flutter/flutter/pull/49815/files#r376168906

[blasten]

The test is always under a test_driver directory. Potentially that could be the heuristic.

[Piinks]

Perfect. SGTM.