JSNumber.toDart will now be two functions: toDartDouble and toDartInt.

There was code that did an Object.toJS. This has been changed to
use Function.toJS as well to make it consistent with the code
in flutter/packages:
https://github.com/flutter/packages/blob/0ef393811d0c2653e68ac135733353fcad8fffa9/packages/web_benchmarks/lib/src/recorder.dart#L1223

This is to help land this CL:
https://dart-review.googlesource.com/c/sdk/+/309082

https://dart-review.googlesource.com/c/sdk/+/309081 is the CL that added
the new methods.

## 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], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.