[dart2js] Ensure generator return types are included in closure RTI need computation.
[dart2js] Ensure generator return types are included in closure RTI need
computation.

This fixes the failure on
https://dart-review.googlesource.com/c/sdk/+/73360.

Change-Id: I5f082307abf7e93038d5c03ee54015c59c6b3564
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/162420
Reviewed-by: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@google.com>
Commit-Queue: Mayank Patke <e0355b78204b67a55a952941c68c8f1b40235938@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk