Just a list of small code clean-ups we should do in the web engine:

• Move test_embedding.dart from lib/ to test/.
• Rename initialization.dart to web_only_api.dart for clarity.
  • This file should be removed entirely: [web] Deprecate and remove all public web-only APIs from dart:ui #127395
• Compile web engine tests with sound null safety.
  • Fixed in [web] Compile tests with sound null safety engine#36169
• Move toMatrix32 from initialization.dart to vector_math.dart.
  • Fixed in [web] move some vector functions into vector_math.dart engine#32782
• webOnlyAssetManager should return a public type (engine.AssetManager is private). Alternatively, we could try removing it, but that's a bigger project.
  • Fixed in [web] Move webOnlyAssetManager to dart:ui_web engine#42642
• felt test should throw an error when asked to test a file without _test.dart suffix. Otherwise, locally you can do felt test test/something.dart and it will pass, but the test won't run on LUCI.
• Convert lineLookup to something with an explicit initialization API, such as initializeLineLookup() + lineLookup pair, like other APIs. Calling a getter for its side-effect is not idiomatic, and it also does not support deinitialization, which we aim to add in the future.
  • Fixed in [web] fix a few analysis lints engine#27375
• Don't use sync as variable name in futurize and _futurize.
  • Fixed in [web] sync => isSync , scuba => golden engine#43699
• Replace the word "scuba" with "golden" in web tests.
  • Fixed in [web] sync => isSync , scuba => golden engine#43699
• Re-evaluate how tests initialize the engine. Currently, most call webOnlyInitializePlatform, some call initializeEngine. We also have test_embedding.dart. It's kind of messy.
• canvas_mask_filter_golden_test.dart goldens are broken on iOS Safari (they are blank).