fix(scope): handle event registration/deregistration on destroyed scope

When trying to register an event on a child scope of an already
destroyed scope, make this a noop
When trying to deregister an event on a child scope of an already
destroyed scope, make this a noop

Closes angular#6897

src/ng/rootScope.js

@@ -921,6 +921,7 @@ function $RootScopeProvider(){
        * @returns {function()} Returns a deregistration function for this listener.
        */
       $on: function(name, listener) {
+        if (this.$$destroyed) return noop;
         var namedListeners = this.$$listeners[name];
         if (!namedListeners) {
           this.$$listeners[name] = namedListeners = [];
@@ -937,6 +938,7 @@ function $RootScopeProvider(){
 
         var self = this;
         return function() {
+          if (self.$$destroyed) return noop;
           namedListeners[indexOf(namedListeners, listener)] = null;
           decrementListenerCount(self, 1, name);
         };

test/ng/rootScopeSpec.js

@@ -874,6 +874,26 @@ describe('Scope', function() {
       $rootScope.$broadcast(EVENT);
       expect(spy.callCount).toBe(1);
     }));
+
+    it('should not throw when trying to listen to an event on a child scope of an already destroyed scope', inject(function($rootScope) {
+      var parent = $rootScope.$new(),
+        child = parent.$new();
+
+      parent.$destroy();
+      expect(function() {
+        var fn = child.$on('someEvent', angular.noop);
+        fn();
+      }).not.toThrow();
+    }));
+
+    it('should not throw when deregistering a listener on a child scope of an already destroyed scope', inject(function($rootScope) {
+      var parent = $rootScope.$new(),
+        child = parent.$new(),
+        fn = child.$on('someEvent', angular.noop);
+
+      parent.$destroy();
+      expect(function() { fn(); }).not.toThrow();
+    }));
   });