This CL fixes type inference of closures using '=>' syntax so that
they infer consistently with an equivalent closure defined using block
syntax ('{}').
Fixes#31436.
Change-Id: I5463dd0ba986e9953ebabbc24ce3ced9a7220007
Reviewed-on: https://dart-review.googlesource.com/27084
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>