anchor.diff:            expect(example.inverse.values, [1, 2, 3]);
             expect(example.inverse.containsValue(2), isTrue);
             expect(example.inverse.containsValue('b'), isFalse);
  +        });
  +        test('iteration', () {
  +          var keys = new List(), values = new List();
  +          example.forEach((key, value) {
  +            keys.add(key);
  +            values.add(value);
  +          });
  +          expect(example.keys, keys);
  +          expect(example.values, values);
           });
         });
         group('writing', () {
  
positive.code:         test('empty', () {
            var target = new BiMap();
            expect(target, isEmpty);
            expect(target, hasLength(0));
          });
          test('copy', () {
  
negative.code:           target.clear();
            expect(target, isEmpty);
            expect(target.inverse, isEmpty);
          });
          test('define if absent', () {
            var target = new BiMap.from(example);
            target.putIfAbsent(1, () => fail('Value already present!'));
            target.putIfAbsent(4, () => 'd');
            expect(target[4], 'd');
          });
        });
      });
  