fix($rootScope): fix potential memory leak when removing scope listeners

When removing listeners they are removed from the array but the array size is not changed
until the event is fired again. If the event is never fired but listeners are added/removed then
the array will continue growing.

By changing the listener removal to `delete` the array entry instead of setting it to `null`
browsers can potentially deallocate the memory for the entry.

Fixes angular#16135
Closes angular#16161

src/ng/rootScope.js

@@ -1207,7 +1207,10 @@ function $RootScopeProvider() {
         return function() {
           var indexOfListener = namedListeners.indexOf(listener);
           if (indexOfListener !== -1) {
-            namedListeners[indexOfListener] = null;
+            // Use delete in the hope of the browser deallocating the memory for the array entry,
+            // while not shifting the array indexes of other listeners.
+            // See issue https://github.com/angular/angular.js/issues/16135
+            delete namedListeners[indexOfListener];
             decrementListenerCount(self, 1, name);
           }
         };

test/ng/rootScopeSpec.js

@@ -1903,6 +1903,21 @@ describe('Scope', function() {
         }));
 
 
+        // See issue https://github.com/angular/angular.js/issues/16135
+        it('should deallocate the listener array entry', inject(function($rootScope) {
+          var remove1 = $rootScope.$on('abc', noop);
+          $rootScope.$on('abc', noop);
+
+          expect($rootScope.$$listeners['abc'].length).toBe(2);
+          expect(0 in $rootScope.$$listeners['abc']).toBe(true);
+
+          remove1();
+
+          expect($rootScope.$$listeners['abc'].length).toBe(2);
+          expect(0 in $rootScope.$$listeners['abc']).toBe(false);
+        }));
+
+
         it('should call next listener after removing the current listener via its own handler', inject(function($rootScope) {
           var listener1 = jasmine.createSpy('listener1').and.callFake(function() { remove1(); });
           var remove1 = $rootScope.$on('abc', listener1);