fix($rootScope): stop memory leak in IE9

DO NOT MERGE
This is a work in progress for discussion - it is not ideal is lacking tests.

Closes #10706

src/ng/rootScope.js

@@ -101,6 +101,15 @@ function $RootScopeProvider() {
         $event.currentScope.$$destroyed = true;
     }
 
+    function cleanUpScope($scope) {
+      // Recurse bottom up
+      $scope.$$childHead && cleanUpScope($scope.$$childHead);
+      $scope.$$nextSibling && cleanUpScope($scope.$$nextSibling);
+
+      $scope.$$nextSibling = $scope.$$prevSibling = $scope.$$childHead =
+          $scope.$$childTail = $scope.$root = $scope.$$watchers = null;
+    }
+
     /**
      * @ngdoc type
      * @name $rootScope.Scope
@@ -900,6 +909,13 @@ function $RootScopeProvider() {
         this.$on = this.$watch = this.$watchGroup = function() { return noop; };
         this.$$listeners = {};
 
+        var that = this;
+        if (msie === 9) {
+          this.$evalAsync(function() {
+            cleanUpScope(that);
+          });
+        }
+
         // All of the code below is bogus code that works around V8's memory leak via optimized code
         // and inline caches.
         //