[web] Add external factory constructors to anonymous canvaskit api classes.

[joshualitt]

This CL adds external factory constructors to many cansvaskit api classes. These are necessary because Dart2Wasm does not support generative constructors on JS interop classes.

For this CL, I've added factory constructors to anonymous classes in the canvaskit API.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[joshualitt]

@eyebrowsoffire ptal, to create this CL I checked each class in canvaskit_api.dart to see if the generative constructor was used in the engine. I might have missed something, but that will be easy to fix later. Next I will fix the use of int.

[eyebrowsoffire]

LGTM!

[Hixie]

test-exemption: code refactor with no semantic change

[joshualitt]

Superseded by #37443