*      function formatter(value) {
 *        if (value) {
 *          return value.toUpperCase();
 *        }
 *      }
 *      ngModel.$formatters.push(formatter);
 *      
 * @property {Object} $error An bject hash with all errors as keys.
 *
 * @property {boolean} $pristine True if user has not interacted with the control yet.
 * @property {boolean} $dirty True if user has already interacted with the control.
 * @property {boolean} $valid True if there is no error.
 * @property {boolean} $invalid True if at least one error on the control.
 *
 * @description
 *
 * `NgModelController` provides API for the `ng-model` directive. The controller contains
 * services for data-binding, validation, CSS updates, and value formatting and parsing. It
 * purposefully does not contain any logic which deals with DOM rendering or listening to
 * DOM events. Such DOM related logic should be provided by other directives which make use of
 * `NgModelController` for data-binding.
 *
 * ## Custom Control Example
 * This example shows how to use `NgModelController` with a custom control to achieve
 * data-binding. Notice how different directives (`contenteditable`, `ng-model`, and `required`)
 * collaborate together to achieve the desired result.
 *
 * Note that `contenteditable` is an HTML5 attribute, which tells the browser to let the element
 * contents be edited in place by the user.  This will not work on older browsers.
 *
 *