Revert "[dart2wasm] Don't dartify JS values when returning as void"

This reverts commit 4bb05dc562c0d5f32436bf31d75d3f96f4677601.

Reason for revert: suspected to be blocking rolls into Flutter, e.g. https://github.com/flutter/flutter/pull/166618

Original change's description:
> [dart2wasm] Don't dartify JS values when returning as void
>
> When calling a JS function that returns `void`, avoid dartifying the
> result.
>
> Technically the return value of `void` functions can still be used, by
> casting the return type to `Object?` or `dynamic`. However this
> shouldn't be done, and `dartifyRaw` overhead just to support this case
> which should be extremely rare is too much.
>
> Any hacky code that uses return values of `void`-returning JS interop
> functions can manually dartify the retrun values with `toDart` or
> similar.
>
> Issue: https://github.com/dart-lang/sdk/issues/60357
> Change-Id: Ic370c7cf6eb6982f61f8a07c91e3bb93c5345ac6
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/417240
> Reviewed-by: Srujan Gaddam <srujzs@google.com>
> Reviewed-by: Martin Kustermann <kustermann@google.com>
> Commit-Queue: Ömer Ağacan <omersa@google.com>

Issue: https://github.com/dart-lang/sdk/issues/60357
Change-Id: I27e0a7355eeee94d1dc02fba0c9569b53954d1c5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/420780
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>
Reviewed-by: Srujan Gaddam <srujzs@google.com>
Commit-Queue: Derek Xu <derekx@google.com>