title: Polymer not changing value of defined property
body: <p>I am using digits-api for login. I am initializing the script on attach using attached life cycle event. </p>
  
  <pre><code>attached: function(){
           console.log("Attached")
           Digits.init({ consumerKey: '************' });
          }
  </code></pre>
  
  <p>Then On clicking the Button it user can login.</p>
  
  <pre><code>onLoginButtonTap : function(){
          Digits.logIn().done(this.onLogIn).fail(this.onFail);
          },
  </code></pre>
  
  <p>When the user successfully login it will go to onLogIn function.</p>
  
  <pre><code>onLogIn: function() {
              console.log("Login Successfull");
              this.loginStatus="Log Out"
          }
  </code></pre>
  
  <p>So what I am doing is changing text inside paper button from Log In to Log out.
  I have a property login status.</p>
  
  <pre><code>loginStatus:{
        type : String,
        value: "Log In",
        notify: true,
        reflectToAttribute: true
        }
  </code></pre>
  
  <p>When I try to change text in onLogIn method using this.loginStatus="Log Out". <strong>It  says Uncaught TypeError: Cannot set property 'loginStatus' of undefined.</strong> So basically I saw this in my other codes too. I am not able to access ids of elements inside function inside a function. Same is the matter here <strong>I am not able to change value of property in function inside a function.</strong> How do I recover from this problem.</p>
  
  <pre><code>&lt;paper-button raised class="custom indigo" id="logIn"&gt;[[loginStatus]]&lt;/paper-button&gt;
  </code></pre>