fix($rootScope): allow destroying a root scope

When running (i.e. bootstrapping and killing) multiple apps on the same page,
it makes sense to destroy the root scope.

Closes angular#11241
Closes angular#10895

Author: vojtajina

src/ng/rootScope.js

@@ -871,13 +871,12 @@ function $RootScopeProvider() {
        * clean up DOM bindings before an element is removed from the DOM.
        */
       $destroy: function() {
-        // we can't destroy the root scope or a scope that has been already destroyed
+        // We can't destroy a scope that has been already destroyed.
         if (this.$$destroyed) return;
         var parent = this.$parent;
 
         this.$broadcast('$destroy');
         this.$$destroyed = true;
-        if (this === $rootScope) return;
 
         incrementWatchersCount(this, -this.$$watchersCount);
         for (var eventName in this.$$listenerCount) {
@@ -886,8 +885,8 @@ function $RootScopeProvider() {
 
         // sever all the references to parent scopes (after this cleanup, the current scope should
         // not be retained by any of our references and should be eligible for garbage collection)
-        if (parent.$$childHead == this) parent.$$childHead = this.$$nextSibling;
-        if (parent.$$childTail == this) parent.$$childTail = this.$$prevSibling;
+        if (parent && parent.$$childHead == this) parent.$$childHead = this.$$nextSibling;
+        if (parent && parent.$$childTail == this) parent.$$childTail = this.$$prevSibling;
         if (this.$$prevSibling) this.$$prevSibling.$$nextSibling = this.$$nextSibling;
         if (this.$$nextSibling) this.$$nextSibling.$$prevSibling = this.$$prevSibling;
 

test/ng/rootScopeSpec.js

@@ -1021,16 +1021,33 @@ describe('Scope', function() {
 
 
     it('should broadcast $destroy on rootScope', inject(function($rootScope) {
-      var spy = spyOn(angular, 'noop');
-      $rootScope.$on('$destroy', angular.noop);
+      var spy = jasmine.createSpy('$destroy handler');
+      $rootScope.$on('$destroy', spy);
       $rootScope.$destroy();
-      $rootScope.$digest();
-      expect(log).toEqual('123');
       expect(spy).toHaveBeenCalled();
       expect($rootScope.$$destroyed).toBe(true);
     }));
 
 
+    it('should remove all listeners after $destroy of rootScope', inject(function($rootScope) {
+      var spy = jasmine.createSpy('$destroy handler');
+      $rootScope.$on('dummy', spy);
+      $rootScope.$destroy();
+      $rootScope.$broadcast('dummy');
+      expect(spy).not.toHaveBeenCalled();
+    }));
+
+
+    it('should remove all watchers after $destroy of rootScope', inject(function($rootScope) {
+      var spy = jasmine.createSpy('$watch spy');
+      var digest = $rootScope.$digest;
+      $rootScope.$watch(spy);
+      $rootScope.$destroy();
+      digest.call($rootScope);
+      expect(spy).not.toHaveBeenCalled();
+    }));
+
+
     it('should remove first', inject(function($rootScope) {
       first.$destroy();
       $rootScope.$digest();