class A {
  foo() => 23;
}
extension B ;
on A;
? { foo() => 42; bar() => 87; }
extension C ;
on A (){ }
extension D ;
on int ;
Function(){ }
? { int call() => 76; }
main() { }
testA(A? a) { }
testFunction(int Function()? f) { }
expect(expected, actual) { }