anchor.diff:      ]);
     }
   
  -  test_postfixInc_null() async {
  -    await assertErrorsInCode(r'''
  -m() {
  -  Null x;
  -  x++;
  -}
  -''', [
  -      error(HintCode.UNUSED_LOCAL_VARIABLE, 13, 1),
  -      error(CompileTimeErrorCode.UNDEFINED_OPERATOR, 19, 2),
  -    ]);
  -  }
  -
  -  test_prefixDec_null() async {
  -    await assertErrorsInCode(r'''
  -m() {
  -  Null x;
  -  --x;
  -}
  -''', [
  -      error(HintCode.UNUSED_LOCAL_VARIABLE, 13, 1),
  -      error(CompileTimeErrorCode.UNDEFINED_OPERATOR, 18, 2),
  -    ]);
  -  }
  -
  -  test_prefixExpression() async {
  -    await assertNoErrorsInCode(r'''
  -class A {}
  -f(var a) {
  -  if (a is A) {
  -    ++a;
  -  }
  -}
  -''');
  -  }
  -
  -  test_prefixExpression_inSubtype() async {
  -    await assertNoErrorsInCode(r'''
  -class A {}
  -class B extends A {
  -  operator +(B b) {return new B();}
  -}
  -f(var a) {
  -  if (a is A) {
  -    ++a;
  -  }
  -}
  -''');
  -  }
  -
     test_prefixExpression_mixin() async {
       await assertErrorsInCode(r'''
   mixin M {}
  
positive.code:     ]);
    }
  
    test_minus_null() async {
      await assertErrorsInCode(r'''
  m() {
    Null x;
    x - 3;
  }
  ''', [
        error(CompileTimeErrorCode.UNDEFINED_OPERATOR, 20, 1),
      ]);
    }
  
    test_minusEq_null() async {
      await assertErrorsInCode(r'''
  m() {
    Null x;
    x -= 1;
  }
  ''', [
        error(HintCode.UNUSED_LOCAL_VARIABLE, 13, 1),
        error(CompileTimeErrorCode.UNDEFINED_OPERATOR, 20, 2),
      ]);
    }
  
    test_plus_null() async {
      await assertErrorsInCode(r'''
  m() {
    Null x;
    x + 3;
  }
  ''', [
        error(CompileTimeErrorCode.UNDEFINED_OPERATOR, 20, 1),
      ]);
    }
  
    test_plus_undefined() async {
      await assertErrorsInCode(r'''
  class A {}
  
negative.code: ''', [
        error(CompileTimeErrorCode.UNDEFINED_OPERATOR, 24, 1),
      ]);
    }
  
    test_postfixExpression_mixin() async {
  