feat(ngForm): Supports expression in form names

<form name="ctrl.form"> form controller will accessible
as $scope.ctrl.form instead of $scope['ctrl.form']

BREAKING CHANGE:
If you have form names that will evaluate as an expression:

<form name="ctrl.form">

And if you are accessing the form from your controller:

  Before:

  function($scope) {
    $scope['ctrl.form'] // form controller instance
  }

  After:

  function($scope) {
    $scope.ctrl.form // form controller instance
  }

This makes it possible to access a form from a controller
using the new "controller as" syntax. Supporting the previous
behavior offers no benefit.

src/ng/directive/form.js

@@ -306,13 +306,13 @@ var formDirectiveFactory = function(isNgForm) {
                 alias = attr.name || attr.ngForm;
 
             if (alias) {
-              scope[alias] = controller;
+              setter(scope, alias, controller, alias);
             }
             if (parentFormCtrl) {
               formElement.bind('$destroy', function() {
                 parentFormCtrl.$removeControl(controller);
                 if (alias) {
-                  scope[alias] = undefined;
+                  setter(scope, alias, undefined, alias);
                 }
                 extend(controller, nullFormCtrl); //stop propagating child destruction handlers upwards
               });

test/ng/directive/formSpec.js

@@ -83,10 +83,11 @@ describe('form', function() {
   });
 
 
-  it('should allow form name to be an expression', function() {
+  it('should support expression in form name', function() {
     doc = $compile('<form name="obj.myForm"></form>')(scope);
 
-    expect(scope['obj.myForm']).toBeTruthy();
+    expect(scope.obj).toBeDefined();
+    expect(scope.obj.myForm).toBeTruthy();
   });
 
 
@@ -325,6 +326,30 @@ describe('form', function() {
     });
 
 
+    it('should deregister a child form whose name is an expression when its DOM is removed', function() {
+      doc = jqLite(
+        '<form name="parent">' +
+          '<div class="ng-form" name="child.form">' +
+          '<input ng:model="modelA" name="inputA" required>' +
+          '</div>' +
+          '</form>');
+      $compile(doc)(scope);
+      scope.$apply();
+
+      var parent = scope.parent,
+        child = scope.child.form;
+
+      expect(parent).toBeDefined();
+      expect(child).toBeDefined();
+      expect(parent.$error.required).toEqual([child]);
+      doc.children().remove(); //remove child
+
+      expect(parent.child).toBeUndefined();
+      expect(scope.child.form).toBeUndefined();
+      expect(parent.$error.required).toBe(false);
+    });
+
+
     it('should deregister a input when its removed from DOM', function() {
       doc = jqLite(
         '<form name="parent">' +