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

When removing listeners the listener is removed from the array but the array size is not changed until the event is fired again. If that event is never fired but listeners are added/removed then this array will continue growing. This changes the listener removal to `delete` the array entry instead of setting it to `null` in the hope of the browser deallocating the memory for the array entry.

Fixes angular#16135

src/ng/rootScope.js

@@ -1180,7 +1180,10 @@ 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

@@ -2316,6 +2316,21 @@ 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 when removing current', inject(function($rootScope) {
           var remove1 = $rootScope.$on('abc', function() { remove1() });