This is a reland of commit 6f4bed30cc
Original change's description:
> [vm] Fix CompileType::CanBeFuture()
>
> In certain cases compiler can omit a type check in 'await e' if it can
> prove that expression 'e' cannot be evaluated to Future.
>
> Previously, if actual type of 'e' is unknown, CompileType::CanBeFuture()
> tested if static type of 'e' is a subtype of Future.
>
> This is not correct, as static type could have a subtype which is also
> a subtype of Future, but static type itself is not a subtype of Future:
>
> class A {}
> class B implements A, Future<A> {}
>
> A e = confuse(B()); // 'e' is B, but has a static type A.
> await e; // A is not a subtype of Future, but 'e' is Future
> // and should be awaited.
>
TEST=language/async/await_flatten_test
Fixes https://github.com/dart-lang/sdk/issues/52585
Change-Id: I37add9021261c95dee5930e031187868e821c161
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/309481
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Alexander Markov
47abd96c5b