Fix a bug in valid return detection

We had implemented the rule for invalid returns in asynchronous
non-generator functions with return type T and a return expression
with static type S:

  is an error if flatten(T) is void and flatten(S) is not void,
  dynamic, or Null

and the actual rule should be:

  is an error if T is void and flatten(S) is not void, dynamic, or
  Null

which requires T to be exactly void and so signals fewer errors.

Change-Id: I296bb8a607d6e5ad862017674091eafe0f6016da
Reviewed-on: https://dart-review.googlesource.com/73740
Commit-Queue: Kevin Millikin <kmillikin@google.com>
Reviewed-by: Leaf Petersen <leafp@google.com>
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>