fix hammer_gestures infinite loop

Author: asukaleido

modules/angular2/src/platform/dom/events/hammer_gestures.ts

@@ -24,9 +24,9 @@ export class HammerGesturesPlugin extends HammerGesturesPluginCommon {
       var mc = new Hammer(element);
       mc.get('pinch').set({enable: true});
       mc.get('rotate').set({enable: true});
-      var handler = function(eventObj) { zone.run(function() { handler(eventObj); }); };
-      mc.on(eventName, handler);
-      return () => { mc.off(eventName, handler); };
+      var callback = function(eventObj) { zone.run(function() { handler(eventObj); }); };
+      mc.on(eventName, callback);
+      return () => { mc.off(eventName, callback); };
     });
   }
 }