title: Angular 1.5 component Expression 'undefined' in attribute
body: <p>I'm trying to use AngularJs 1.5 components but everytime I declare <code>bindings</code> I keep getting this error:</p>
  
  <blockquote>
    <p>Expression 'undefined' in attribute 'message' used with directive 'homeComp' is non-assignable!</p>
  </blockquote>
  
  <p>I'm trying a simple component, just to learn it, this is the code:</p>
  
  <pre><code>var component = {
    bindings: {
        message: '='
    },
    controllerAs: 'vm',
    controller: function MainController() {
      this.message = 'Welcome to my component';
  
      function debug() {
        this.message = 'This message changed';
      }
      this.debug = debug;
    },
    template: [
      'Message: {{ vm.message }}&lt;br /&gt;',
      '&lt;button ng-click="vm.debug()"&gt;Change message&lt;/button&gt;'
    ].join(``)
  };
  </code></pre>
  
  <p>You can see the error here: <a href="http://plnkr.co/edit/uutk5kxOVpa5eLfjoa8U?p=preview" rel="nofollow">http://plnkr.co/edit/uutk5kxOVpa5eLfjoa8U?p=preview</a></p>
  
  <p>What is wrong with the code? Or what is causing this error? If I remove the bindings, the error doesn't appear and I can change the message.</p>