8491586ef9
Any user @staticInterop types should only subtype the dart:js_interop types JSObject and or JSAny as user @staticInterop types erase to JSObject. In the future, the other JS types will be added as extension types, allowing users to implement them with their own extension types. Allows https://dart-review.googlesource.com/c/sdk/+/316865/1 to be landed. ## Pre-launch Checklist - [X] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [X] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [X] I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides]. - [X] I listed at least one issue that this PR fixes in the description above. - [ ] I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on writing and running engine tests. - [X] I updated/added relevant documentation (doc comments with `///`). - [X] I signed the [CLA]. - [ ] All existing and new tests are passing.