Uncaught AssertionError: Assertion failed: Event target is not initialized. Did you call superclass (goog.events.EventTarget) constructor? 

Fix the error:
Uncaught AssertionError: Assertion failed: Event target is not initialized. Did you call superclass (goog.events.EventTarget) constructor? 

Occurs when debugging not compiled code. 

MapBrowserEventHandler should have the  eventTargetListeners_ implemented because it  is neseccary to pass the assert in goog.events.EventTarget.prototype.assertInitialized