refactor($scope): prevent multiple calls to listener on $destroy

Prevent isolated scopes from having listeners that get called
multiple times when on `$destroy`

Author: lgalfaso

src/ng/rootScope.js

@@ -196,6 +196,7 @@ function $RootScopeProvider(){
           // ensure that there is just one async queue per $rootScope and its children
           child.$$asyncQueue = this.$$asyncQueue;
           child.$$postDigestQueue = this.$$postDigestQueue;
+          child.$on('$destroy', function() { child.$$destroyed = true; });
         } else {
           // Only create a child scope class if somebody asks for one,
           // but cache it to allow the VM to optimize lookups.

test/ng/rootScopeSpec.js

@@ -994,6 +994,13 @@ describe('Scope', function() {
       expect(log).toBe('123');
     }));
 
+    it('should broadcast the $destroy only once', inject(function($rootScope, log) {
+      var isolateScope = first.$new(true);
+      isolateScope.$on('$destroy', log.fn('event'));
+      first.$destroy();
+      isolateScope.$destroy();
+      expect(log).toEqual('event');
+    }));
 
     it('should decrement ancestor $$listenerCount entries', inject(function($rootScope) {
       var EVENT = 'fooEvent',