anchor.diff:            expect(target.keys, ['a', 'b', 'c']);
             expect(target.values, [1, 2, 3]);
           });
  +        test('iterables (error)', () {
  +          expect(() => new BiMap.fromIterables([1], []), throwsArgumentError);
  +          expect(() => new BiMap.fromIterables([], [1]), throwsArgumentError);
  +        });
         });
         group('accessing', () {
           test('indexed', () {
  
positive.code:           target.clear();
            expect(target, isEmpty);
            expect(target.inverse, isEmpty);
          });
        });
      });
  
negative.code:         test('empty', () {
            var target = new BiMap();
            expect(target, isEmpty);
            expect(target.isEmpty, isTrue);
            expect(target.isNotEmpty, isFalse);
            expect(target, hasLength(0));
          });
          test('idenity', () {
            var target = new BiMap.identity();
            expect(target, isEmpty);
            expect(target.isEmpty, isTrue);
            expect(target.isNotEmpty, isFalse);
            expect(target, hasLength(0));
          });
          test('copy', () {
  