[kernel] Allow bare return in sync* and async* functions in verifier
[kernel] Allow bare return in sync* and async* functions in verifier

Such return statements are allowed in Dart, and they can be present in
kernel that has not been processed by the async transformation.

Change-Id: I8592cce58cb8fca2d78cdb0d5e5532715553c1ac
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/242544
Reviewed-by: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@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