Don't infer types when there's an irreconcilable type mismatch.

Also fix the type inference algorithm so that when it explores
multiple alternatives (as a result of the use of FutureOr<>), if one
alternative generated some constraints but failed to produce a match,
it rewinds those constraints and tries the other alternative.
(Previously, it erroneously assumed that if any constraints were
generated, the alternative matched successfully).

Also add unit tests to verify the proper operation of the subtype
match algorithm.

Fixes #32305.

Change-Id: I060b5d6d5247a68d2b27bba78819bae172e43d97
Reviewed-on: https://dart-review.googlesource.com/53685
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Jenny Messerly <[email protected]>

Author: stereotype441