Restore `promiseToFuture` in public API of `dart:html`

[natebosch]

When I committed 0f1d5fa it removed promiseToFuture from the public API. I'm not sure we ever intended for this to be public, but since it is it would be breaking to remove it.

[natebosch]

cc @vsmenon - we should consider this a release blocker

Fix is https://dart-review.googlesource.com/c/sdk/+/120935

[muhleder]

This is currently breaking firebase for web. googlearchive/firebase-dart#251