code: function _beginChange(object, changeType) {
          var notifier = Object.getNotifier(object),
              activeChanges = activeChangesMap.get(notifier),
              changeCount = activeChangesMap.get(notifier)[changeType];
          activeChanges[changeType] = typeof changeCount === 'undefined' ? 1 : changeCount + 1;
      }
docs: Implementation of the internal algorithm 'BeginChange' described in the proposal. [Corresponding Section in ECMAScript wiki](http://wiki.ecmascript.org/doku.php?id=harmony:observe_internals#beginchange)