anchor.diff:          test('invalid', () {
             var set = new Multiset.from(['a', 'a', 'b', 'b', 'b']);
             expect(() => set.remove('c'), returnsNormally);
  -          expect(() => set.remove(false), returnsNormally);
  +          expect(() => set.remove('z'), returnsNormally);
             expect(set, isNot(isEmpty));
             expect(set, hasLength(5));
             expect(set, unorderedEquals(['a', 'a', 'b', 'b', 'b']));
  
positive.code:           expect(target.values, [2, 3, 4]);
          });
          test('iterables', () {
            var target = new BiMap.fromIterables(example.keys, example.values);
            expect(target.keys, [1, 2, 3]);
            expect(target.values, ['a', 'b', 'c']);
            target = new BiMap.fromIterables(example.values, example.keys);
            expect(target.keys, ['a', 'b', 'c']);
            expect(target.values, [1, 2, 3]);
          });
          test('iterables (error)', () {
            expect(() => new BiMap.fromIterables([1], []), throwsArgumentError);
  
negative.code:         });
          test('define if absent', () {
            var target = new BiMap.from(example);
            target.putIfAbsent(1, () {
              fail('Value already present!');
              return null;
            });
            target.putIfAbsent(4, () => 'd');
            expect(target[4], 'd');
          });
  