dart:html should add type descriptions for JS foreign functions in promiseToFuture calls

[srujzs]

In many cases, promiseToFuture calls to get promises on a native type look like the following:

promiseToFuture<RtcSessionDescription>(JS("", "#.createOffer(#)", this, options_dict))

This is an issue since RtcSessionDescription is not necessarily live at the time and will fail to type-check. In order to circumvent this, and make the type live, there needs to exist a type description in the JS foreign function that signals to the compiler that this type may be live after this call, e.g.

JS("creates:<type>;", ...)

For more context, see #42796 .