[js_runtime] Polyfill WeakReference and Finalizer for ancient browsers

Dart does not officially support browsers before the introduction of WeakRef and FinalizationRegistry but there are users of Dart that have a few customers still using these browsers.

As a result, both ACX and Flutter have ad-hoc polyfills for these classes. Adding the polyfill in js_runtime will allow the other polyfills to be removed. The polyfill is as small as possible (omitting checks, avoiding extra Dart classes, detecting as late as possible), so it should be a net code size improvement of a few bytes for ACX and Flutter use cases.

The polyfill leaks memory by retaining the weak reference target and never calls the finalization callbacks. This is permitted behaviour.

Change-Id: I4f0d08ee6322dab26728d95ca8c24a7fd59b3314
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/333304
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Stephen Adams <sra@google.com>