title: Not able to reference Observable Class fields from xml layout
body: <p>I'm looking at the new Data Binding plugin for Android and trying to integrate it in a project.</p>
  
  <p>While reading about creating observable objects, i encountered the ObservableFields documentation that involves using self-contained observable objects example from documentation :</p>
  
  <pre><code>public class User extends BaseObservable {
     public final ObservableField&lt;String&gt; firstName =
         new ObservableField&lt;&gt;();
     public final ObservableField&lt;String&gt; lastName =
         new ObservableField&lt;&gt;();
     public final ObservableInt age = new ObservableInt();
  }
  </code></pre>
  
  <p>Above snippet would be a replacement of:</p>
  
  <pre><code>private static class User extends BaseObservable {
     private String firstName;
     private String lastName;
     @Bindable
     public String getFirstName() {
         return this.firstName;
     }
     @Bindable
     public String getFirstName() {
         return this.lastName;
     }
     public void setFirstName(String firstName) {
         this.firstName = firstName;
         notifyPropertyChanged(BR.firstName);
     }
     public void setLastName(String lastName) {
         this.lastName = lastName;
         notifyPropertyChanged(BR.lastName);
     }
  }
  </code></pre>
  
  <p>Which is significantly less code; But when using Observable Fields and reference then from the XML layout like such:</p>
  
  <pre><code>&lt;layout xmlns:android="http://schemas.android.com/apk/res/android"&gt;
     &lt;data&gt;
         &lt;variable name="user" type="com.example.User"/&gt;
     &lt;/data&gt;
     &lt;LinearLayout
         android:orientation="vertical"
         android:layout_width="match_parent"
         android:layout_height="match_parent"&gt;
  
         &lt;TextView android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:text="@{user.lastName}"
             android:id="@+id/lastName"/&gt;
  
     &lt;/LinearLayout&gt;
  &lt;/layout&gt;
  </code></pre>
  
  <p>I am getting this error:</p>
  
  <pre><code>../../databinding/ActivityMainBinding.java
  Error:(127, 20) error: cannot find symbol variable lastName 
  </code></pre>
  
  <p>And that is not verbose at all; 
  Not using ObservableFields works like a charm.</p>
  
  <p><a href="https://developer.android.com/tools/data-binding/guide.html#observable_objects" rel="noreferrer">Documentation reference</a></p>
  
  <p>Anyone facing this issue ? Thoughts?</p>