message: fix(core): __symbol__ should return __zone_symbol__ without zone.js loaded
diff: @@ -22,8 +22,8 @@ import {EventManagerPlugin} from './event_manager';
  * addEventListener by 3x.
  */
  const __symbol__ =
  - (typeof Zone !== 'undefined') && (Zone as any)['__symbol__'] || function<T>(v: T): T {
  - return v;
  + (typeof Zone !== 'undefined') && (Zone as any)['__symbol__'] || function(v: string): string {
  + return '__zone_symbol__' + v;
  };
  const ADD_EVENT_LISTENER: 'addEventListener' = __symbol__('addEventListener');
  const REMOVE_EVENT_LISTENER: 'removeEventListener' = __symbol__('removeEventListener');
  