fix(scope): do not remove the listeners properties on destroy

Destroying the listener properties cases many issues when any of these
operations is being performed on a destroyed scope or a child scope
* Registering a new event
* Deregistering an existing event
* Destroying an insolated child scope with a destroyed parent
* Sending an `$emit` from a child scope with a destroyed parent

Closes angular#6897

src/ng/rootScope.js

@@ -745,12 +745,12 @@ function $RootScopeProvider(){
         // - https://github.com/angular/angular.js/issues/6794#issuecomment-38648909
         // - https://github.com/angular/angular.js/issues/1313#issuecomment-10378451
         for (var prop in this) {
-          if (hasOwnProperty.call(this, prop)) {
+          if (hasOwnProperty.call(this, prop) && prop !== '$$destroyed' &&
+              // Avoid removing the listener properties as this causes many issues like #6897
+              prop !== '$$listeners' && prop !== '$$listenerCount') {
             this[prop] = null;
           }
         }
-        // recreate the $$destroyed flag
-        this.$$destroyed = true;
       },
 
       /**

test/ng/rootScopeSpec.js

@@ -874,6 +874,64 @@ 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.$destroy();
+      expect(function() { fn(); }).not.toThrow();
+    }));
+
+    it('should not throw when trying to destroy a child insolated scope of an already destroyed scope', inject(function($rootScope) {
+      var parent = $rootScope.$new(),
+        child = parent.$new(true),
+        fn = child.$on('someEvent', angular.noop);
+
+      parent.$destroy();
+      expect(function() { child.$destroy(); }).not.toThrow();
+    }));
+
+    it('should not throw when trying to listen to an event on an insolated child scope of an already destroyed scope', inject(function($rootScope) {
+      var parent = $rootScope.$new(),
+        child = parent.$new(true);
+
+      parent.$destroy();
+      expect(function() {
+        var fn = child.$on('someEvent', angular.noop);
+        fn();
+      }).not.toThrow();
+    }));
+
+    it('should not throw when deregistering a listener on an insolated child scope of an already destroyed scope', inject(function($rootScope) {
+      var parent = $rootScope.$new(),
+        child = parent.$new(true),
+        fn = child.$on('someEvent', angular.noop);
+
+      parent.$destroy();
+      expect(function() { fn(); }).not.toThrow();
+    }));
+
+    it('should not throw when trying to emit from an insolated child scope of a destroyed scope', inject(function($rootScope) {
+      var parent = $rootScope.$new(),
+        child = parent.$new(true),
+        fn = child.$on('someEvent', angular.noop);
+
+      parent.$destroy();
+      expect(function() { child.$emit('someEvent'); }).not.toThrow();
+    }));
   });