Incorrect "unnecessary cast" hint.

[leafpetersen]

The following program produces an "unnecessary hint" cast from the analyzer.

void main() {
  Function(Null) f = (String x) => x;
  (f as Function(int))(3); 
}

The cast here is definitely necessary.

[leafpetersen]

This showed up again, causing some confusion.

[leafpetersen]

This popped up again fixing internal code with a slightly different repro:

class A {}

class B<T extends A> {
  void foo() {
    T Function(T) f;
    A Function(A) g;
    g = f as A Function(A);
  }
}

I suspect that the unnecessary cast code might be using Dart 1 subtyping.

cc @vsmenon @bwilkerson

[bwilkerson]

https://dart-review.googlesource.com/c/sdk/+/54715