fix(ngModelOptions): preserve context of getter/setters

Closes #9394

BREAKING CHANGE: previously, ngModel invoked getter/setters in the global context.

For example:

```js
<input ng-model="model.value" ng-model-options="{ getterSetter: true }">
```

would previously invoke `model.value()` in the global context.

Now, ngModel invokes `value` with `model` as the context.

It's unlikely that real apps relied on this behavior. If they did they can use `.bind` to explicilty
bind a getter/getter to the global context, or just reference globals normally without `this`.

src/ng/directive/input.js

@@ -1740,13 +1740,14 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
 
 
   var parsedNgModel = $parse($attr.ngModel),
+      parsedNgModelContext = null,
       pendingDebounce = null,
       ctrl = this;
 
   var ngModelGet = function ngModelGet() {
     var modelValue = parsedNgModel($scope);
     if (ctrl.$options && ctrl.$options.getterSetter && isFunction(modelValue)) {
-      modelValue = modelValue();
+      modelValue = $scope.$eval($attr.ngModel + '()');
     }
     return modelValue;
   };
@@ -1755,8 +1756,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
     var getterSetter;
     if (ctrl.$options && ctrl.$options.getterSetter &&
         isFunction(getterSetter = parsedNgModel($scope))) {
-
-      getterSetter(ctrl.$modelValue);
+      $scope.$eval($attr.ngModel + '($$$p)', {$$$p: ctrl.$modelValue});
     } else {
       parsedNgModel.assign($scope, ctrl.$modelValue);
     }
@@ -1765,6 +1765,11 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
   this.$$setOptions = function(options) {
     ctrl.$options = options;
 
+    if (ctrl.$options && ctrl.$options.getterSetter && ctrl.$options.getterSetterContext) {
+      // Use the provided context expression to specify the context used when invoking the
+      // getter/setter function
+      parsedNgModelContext = $parse(ctrl.$options.getterSetterContext);
+    }
     if (!parsedNgModel.assign && (!options || !options.getterSetter)) {
       throw $ngModelMinErr('nonassign', "Expression '{0}' is non-assignable. Element: {1}",
           $attr.ngModel, startingTag($element));

test/ng/directive/inputSpec.js

@@ -2093,6 +2093,43 @@ describe('input', function() {
           'ng-model-options="{ getterSetter: true }" />');
     });
 
+    it('should invoke a model in the correct context if getterSetter is true', function() {
+      compileInput(
+        '<input type="text" ng-model="someService.getterSetter" '+
+          'ng-model-options="{ getterSetter: true }" />');
+
+      scope.someService = {
+        value: 'a',
+        getterSetter: function(newValue) {
+          this.value = newValue || this.value;
+          return this.value;
+        }
+      };
+      spyOn(scope.someService, 'getterSetter').andCallThrough();
+      scope.$apply();
+
+      expect(inputElm.val()).toBe('a');
+      expect(scope.someService.getterSetter).toHaveBeenCalledWith();
+      expect(scope.someService.value).toBe('a');
+
+      changeInputValueTo('b');
+      expect(scope.someService.getterSetter).toHaveBeenCalledWith('b');
+      expect(scope.someService.value).toBe('b');
+
+      scope.someService.value = 'c';
+      scope.$apply();
+      expect(inputElm.val()).toBe('c');
+      expect(scope.someService.getterSetter).toHaveBeenCalledWith();
+    });
+
+    it('should fail to parse if getterSetterContext is an invalid expression', function() {
+      expect(function() {
+        compileInput(
+          '<input type="text" ng-model="someService.getterSetter" '+
+            'ng-model-options="{ getterSetter: true, getterSetterContext: \'throw error\' }" />');
+      }).toThrowMinErr("$parse", "syntax", "Syntax Error: Token 'error' is an unexpected token at column 7 of the expression [throw error] starting at [error].");
+    });
+
     it('should assign invalid values to the scope if allowInvalid is true', function() {
       compileInput('<input type="text" name="input" ng-model="value" maxlength="1" ' +
                    'ng-model-options="{allowInvalid: true}" />');