Analyzer flattening futures incorrectly

[fishythefish]

class Divergent<T> implements Future<Divergent<Divergent<T>>> {
  noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
}

void main() {
  Future<Divergent<Divergent<int>>> x = (() async => Divergent<int>())();
}

This code compiles with CFE, but analyzer complains that:

• the return type of the closure is Divergent<int>, which isn't a Future<Divergent<Divergent<Divergent<int>>>> and
• a value of type Future<Divergent<Divergent<Divergent<int>>>> can't be assigned to a variable of type Future<Divergent<Divergent<int>>>.

flatten(Divergent<T>) = Divergent<Divergent<T>>, so the return type of the closure should be Future<Divergent<Divergent<int>>>.

Also see #25611, #31887, #32881.

[stereotype441]

a554c8b introduces a failing test case for this

[srawlins]

This bug is still active today.