This reverts commit 16fcfe7eae.
Reason for revert: Flutter changes haven't landed to google3 yet.
Original change's description:
> [dart:js_interop] Remove Object.toJS and JSNumber.toDart
>
> Modifies JSBoxedDartObject reified type on JS backends and also
> modifies JSBoxedDartObject.toDart now that a proper box is introduced.
> Also uses a JS symbol in JSBoxedDartObject.toDart for a property
> check so that different Dart apps don't accidentally share Dart
> objects. It's now an error to call this function on non-boxed objects.
>
> Fixes a type issue where creating a new object literal with the JS
> foreign function was resulting in dart2js thinking toJSBox would
> always throw. Changing the typeDescription to PlainJavaScriptObject
> instead of =Object fixes that issue.
>
> CoreLibraryReviewExempt: Backend-specific library.
> Change-Id: I5cfb1f32ff4328fafdf9831b0d8da806c39391d9
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/309082
> Reviewed-by: Joshua Litt <joshualitt@google.com>
> Commit-Queue: Srujan Gaddam <srujzs@google.com>
Change-Id: I469ad04db7b49ffef47a46ccac97e909e4865719
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/313580
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Joshua Litt <joshualitt@google.com>
Commit-Queue: Srujan Gaddam <srujzs@google.com>
2767fca6fe