fix(ngModelOptions): preserve context of getter/setters

Many thanks to @NevilleS and @jbedard for collaborating with me on a solution to this!

Closes #9394
Closes #9865

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

@@ -1751,32 +1751,33 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
 
 
   var parsedNgModel = $parse($attr.ngModel),
+      parsedNgModelAssign = parsedNgModel.assign,
+      ngModelGet = parsedNgModel,
+      ngModelSet = parsedNgModelAssign,
       pendingDebounce = null,
       ctrl = this;
 
-  var ngModelGet = function ngModelGet() {
-    var modelValue = parsedNgModel($scope);
-    if (ctrl.$options && ctrl.$options.getterSetter && isFunction(modelValue)) {
-      modelValue = modelValue();
-    }
-    return modelValue;
-  };
-
-  var ngModelSet = function ngModelSet(newValue) {
-    var getterSetter;
-    if (ctrl.$options && ctrl.$options.getterSetter &&
-        isFunction(getterSetter = parsedNgModel($scope))) {
-
-      getterSetter(ctrl.$modelValue);
-    } else {
-      parsedNgModel.assign($scope, ctrl.$modelValue);
-    }
-  };
-
   this.$$setOptions = function(options) {
     ctrl.$options = options;
+    if (options && options.getterSetter) {
+      var invokeModelGetter = $parse($attr.ngModel + '()'),
+          invokeModelSetter = $parse($attr.ngModel + '($$$p)');
 
-    if (!parsedNgModel.assign && (!options || !options.getterSetter)) {
+      ngModelGet = function($scope) {
+        var modelValue = parsedNgModel($scope);
+        if (isFunction(modelValue)) {
+          modelValue = invokeModelGetter($scope);
+        }
+        return modelValue;
+      };
+      ngModelSet = function($scope, newValue) {
+        if (isFunction(parsedNgModel($scope))) {
+          invokeModelSetter($scope, {$$$p: ctrl.$modelValue});
+        } else {
+          parsedNgModelAssign($scope, ctrl.$modelValue);
+        }
+      };
+    } else if (!parsedNgModel.assign) {
       throw $ngModelMinErr('nonassign', "Expression '{0}' is non-assignable. Element: {1}",
           $attr.ngModel, startingTag($element));
     }
@@ -2189,7 +2190,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
     }
     if (isNumber(ctrl.$modelValue) && isNaN(ctrl.$modelValue)) {
       // ctrl.$modelValue has not been touched yet...
-      ctrl.$modelValue = ngModelGet();
+      ctrl.$modelValue = ngModelGet($scope);
     }
     var prevModelValue = ctrl.$modelValue;
     var allowInvalid = ctrl.$options && ctrl.$options.allowInvalid;
@@ -2217,7 +2218,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
   };
 
   this.$$writeModelToScope = function() {
-    ngModelSet(ctrl.$modelValue);
+    ngModelSet($scope, ctrl.$modelValue);
     forEach(ctrl.$viewChangeListeners, function(listener) {
       try {
         listener();
@@ -2313,7 +2314,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
   //       ng-change executes in apply phase
   // 4. view should be changed back to 'a'
   $scope.$watch(function ngModelWatch() {
-    var modelValue = ngModelGet();
+    var modelValue = ngModelGet($scope);
 
     // if scope model value and ngModel value are out of sync
     // TODO(perf): why not move this to the action fn?

test/ng/directive/inputSpec.js

@@ -2102,7 +2102,7 @@ describe('input', function() {
       expect(inputElm.val()).toBe('a');
     });
 
-    it('should always try to invoke a model if getterSetter is true', function() {
+    it('should try to invoke a function model if getterSetter is true', function() {
       compileInput(
         '<input type="text" ng-model="name" ' +
           'ng-model-options="{ getterSetter: true }" />');
@@ -2117,6 +2117,12 @@ describe('input', function() {
       expect(inputElm.val()).toBe('b');
       expect(spy).toHaveBeenCalledWith('a');
       expect(scope.name).toBe(spy);
+    });
+
+    it('should assign to non-function models if getterSetter is true', function() {
+      compileInput(
+        '<input type="text" ng-model="name" ' +
+          'ng-model-options="{ getterSetter: true }" />');
 
       scope.name = 'c';
       changeInputValueTo('d');
@@ -2136,6 +2142,35 @@ 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 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}" />');