Add informal spec for invalid returns.
Add informal spec for invalid returns.

This mostly documents and regularizes existing (post bug fix) semantics,
with a small additional restriction on returning `void` from an async
function.

Change-Id: Ic1a302aeb666fa73ca2b7e23072394490bb4fe76
Reviewed-on: https://dart-review.googlesource.com/60401
Reviewed-by: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>

dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk