feat(scope): broadcast $destroy event on scope destruction

perf testing shows that in chrome this change adds 5-15% overhead
when destroying 10k nested scopes where each scope has a $destroy listener

src/service/scope.js

@@ -136,7 +136,6 @@ function $RootScopeProvider(){
       this.$$phase = this.$parent = this.$$watchers =
                      this.$$nextSibling = this.$$prevSibling =
                      this.$$childHead = this.$$childTail = null;
-      this.$destructor = noop;
       this['this'] = this.$root =  this;
       this.$$asyncQueue = [];
       this.$$listeners = {};
@@ -458,6 +457,8 @@ function $RootScopeProvider(){
         if (this.$root == this) return; // we can't remove the root node;
         var parent = this.$parent;
 
+        this.$broadcast('$destroy');
+
         if (parent.$$childHead == this) parent.$$childHead = this.$$nextSibling;
         if (parent.$$childTail == this) parent.$$childTail = this.$$prevSibling;
         if (this.$$prevSibling) this.$$prevSibling.$$nextSibling = this.$$nextSibling;

test/service/scopeSpec.js

@@ -2,6 +2,9 @@
 
 describe('Scope', function() {
 
+  beforeEach(module(provideLog));
+
+
   describe('$root', function() {
     it('should point to itself', inject(function($rootScope) {
       expect($rootScope.$root).toEqual($rootScope);
@@ -393,6 +396,15 @@ describe('Scope', function() {
       $rootScope.$digest();
       expect(log).toEqual('12');
     }));
+
+
+    it('should broadcast the $destroy event', inject(function($rootScope, log) {
+      first.$on('$destroy', log.fn('first'));
+      first.$new().$on('$destroy', log.fn('first-child'));
+
+      first.$destroy();
+      expect(log).toEqual('first; first-child');
+    }));
   });