anchor.diff:  library unittestTests;
   import 'package:unittest/unittest.dart';
   import 'dart:async';
  +import 'dart:collection';
   part 'test_utils.dart';
   
   doesNotThrow() {}
  
positive.code:     });
  
      test('equals with different lengths', () {
        var a = new Map();
        a['foo'] = 'bar';
        var b = new Map();
        b['foo'] = 'bar';
        b['bar'] = 'foo';
        var c = new Map();
        c['bar'] = 'foo';
        c['barrista'] = 'caffeine';
        shouldFail(a, equals(b),
            "Expected: <{bar: foo, foo: bar}> "
            "but: different map lengths; missing map key 'bar'.");
        shouldFail(b, equals(a),
            "Expected: <{foo: bar}> "
  
negative.code:           "Expected: <{bar: foo, barrista: caffeine}> "
            "but: missing map key 'barrista'.");
        shouldFail(c, equals(b),
            "Expected: <{foo: bar, bar: foo}> "
            "but: missing map key 'foo'.");
        shouldFail(a, equals(c),
            "Expected: <{bar: foo, barrista: caffeine}> "
  