In general any async function can return X or Future<X>. Though the
future implementation has to do different things depending on which case
we're in. It does so by using a `<obj> is Future<T>` test. This test is very
expensive if many different classes flow into `<obj>`.
Though most functions do not return `Future` objects from async
functions. We can determine that statically by looking if any returned
expression could be a future. If not, we can bypass the `is Future<T>`
test entirely.
Issue https://github.com/dart-lang/sdk/issues/48226
Issue https://github.com/dart-lang/sdk/issues/48235
TEST=pkg/front_end/testcases/general/async_function_returns_future_or.dart
Change-Id: If655bdbdddc214dd7b12be9905b3c788252547d0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/230662
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>
Martin Kustermann
4eaacc3df9