title: Two-way binding does not work when using new bindToController syntax
body: <p>I'm trying to bind a variable inside a directive and modify it from the directive using the new Angular 1.4 bindToController syntax.</p>
  
  <p>HTML:</p>
  
  <pre><code>&lt;body ng-app="mainModule" ng-controller="mainController"&gt;
     &lt;h2&gt;{{boundVar}}&lt;/h2&gt;
  &lt;my-directive ng-model="boundVar"&gt;&lt;/my-directive&gt;
  </code></pre>
  
  <p>Javascript:</p>
  
  <pre><code>angular.module('directiveModule', []).directive('myDirective',
  function()
  {
      var r = 
      {
          scope: true,
          bindToController: { value: '=ngModel' },
          template: '&lt;h2&gt;The value is: {{ctrl.value}}&lt;/h2&gt;&lt;br&gt;&lt;button ng-click="ctrl.increment()"&gt;Increment&lt;/button&gt;',
          controllerAs: 'ctrl',
          controller: function()
          {
              this.increment = function() { this.value++; };
          }
      };
  
      return r;
  });
  
  angular.module('mainModule', ['directiveModule']).controller('mainController',
  function($scope)
  {
      $scope.boundVar = 10;
  });
  </code></pre>
  
  <p>When the page is initially load, you see the variable 'boundVar' as effectively been bound:</p>
  
  <p><img src="https://i.stack.imgur.com/upayB.png" alt="enter image description here"></p>
  
  <p>But when you press the button, only the inner variable changes, so the two-way binding appears not to work:</p>
  
  <p><img src="https://i.stack.imgur.com/IUx51.png" alt="enter image description here"></p>
  
  <p>What am I missing?</p>
  
  <p>Thank you!</p>