Improve diagnostic situation for constructor tearoffs from deferred library

[parlough]

Consider:

other.dart:

class C {}

main.dart:

import 'other.dart' deferred as other;

void main() {
  test(other.C.new);
}

void test(Object Function() f) {
  print(f);
}

This provides no analyzer diagnostics (which is a known issue that applies to more situations), but the front end does error:

main.dart:4:16:
Error: 'new' can only be used as a constructor reference.
  test(other.C.new);
               ^^^
Error: Compilation failed.

Most deferred errors will happen at runtime if you haven't imported the library with an explanation as to why, but this is happening at compile time, I'm assuming because the tear off is constant?

It's nice for it to happen at compile time, but the error isn't helpful for developers trying to resolve it since .new is a constructor reference.

Side note: I discovered this issue since unnecessary_lambdas made this change automatically (fix here).