Reland "[vm] Avoid type parameter check of _Future._asyncComplete() when caller knows its safe"

Currently we have to do a runtime `as FutureOr<T>` check for the
parameter to `_Future<T>.asyncComplete(FutureOr<T> value)`. Although
needed for general cases, for the particular usage in the VM's
async/await transformer, we know that the test is going to suceed.

This CL adds two VM-specific methods on `_Future` that take `dynamic`
and downcast via `unsafeCast<>()` to avoid this rather large runtime
type checking overhead.

Issue https://github.com/dart-lang/sdk/issues/48226

Change after revert: Replace the
    assert(value is T || value is Future<T>);
with
    assert((value as FutureOr<T>) == value);

The former doesn't allow `null` while the ladder might (depending
on nullability of `T`).

TEST=ci

Change-Id: I2379c625003fea6aa836ac74beb1cd59201b3560
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/231704
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>