`Add await` quick-fix false-positive

[FMorschel]

Add discarded_futures to analysis_options.yaml or unawaited_futures and async to foo:

Repro:

class A {
  Future<void> m() async {}
}

class B {
  B(this.a);
  final A a;
}

void foo(B b) {
  final _ = b.a..m(); // m() triggers
  final _ = b..a.m(); // m() triggers
}

[bwilkerson]

In the two given cases, the simplest fix is to not offer the assist (as suggested by the title). Longer term we might consider adding a fix that can convert the cascade to a non-cascade and then add the await.

However, while investigating I found a third case. Add the following at the end of foo (a manual conversion to a non-cascade form):

  final a = b.a;
  a.m();

The fix will be suggested on the second line, but applying the fix will produce garbage:

  final a = b.a;
  a.await m();

This case also needs to be fixed to instead produce

  final a = b.a;
  await a.m();

[FMorschel]

Thanks. I'll review all cases.

Longer term we might consider adding a fix that can convert the cascade to a non-cascade and then add the await.

Once #61042 is closed, we can make some progress on this.