anchor.diff:      // '(String, [dynamic]) -> Expression'.
       await assertNoErrorsInCode(r'''
   class I {
  -  int call([int x]) => 0;
  +  int call([int x = 7]) => 0;
   }
   class C implements I {
     noSuchMethod(_) => null;
  
positive.code:   test_staticVariable() async {
      await assertErrorsInCode(r'''
  class A {
    static int x;
  }
  f(var y) {
    if (y is String) {
  
negative.code: }
  
  class C {
    A a = A();
  }
  
  f(C c) {
  