dart:js_interop and package:js will start conflicting, since they both have an `@JS` annotation. Until we're ready to only use dart:js_interop (which will require updating the SDK constraints of every package), we should hide the `@JS` annotation from dart:js_interop. Due to shadowing, this is the behavior today, so there should be no functional change.

Unblocks https://dart-review.googlesource.com/c/sdk/+/294130/8 and prevents confusing shadowing of dart:js_interop annotations like we do today.

- [Mentioned CL that is unblocked] I listed at least one issue that this PR fixes in the description above.
- [Need test-exemption] I added new tests to check the change I am making, or this PR is [test-exempt].
- [Need to run] All existing and new tests are passing.