CFE: Propagate dynamic as the future value type

[eernstg]

Cf. dart-lang/language#997, a return type of dynamic, dynamic?, or dynamic* is now propagated such that the future value type will be dynamic (before that spec change, it was specified to be Object?, which caused unintended breakage).

This issue exists in order to keep track of the implementation changes corresponding to this spec change. The CFE currently (commit 24c50fb) does not raise an error for the following example:

f() async {
  return;
}

void main() async {
  await f();
}

which may indicate that the change has already been implemented, or that neither dart-lang/language#997 nor an earlier change introducing the future value type have been implemented (or something similar to that). So the work may or may not have been completed already.