title: Binding to a non class variable fails in ReactiveUI
body: <p>Given a sample ContentPage</p>
  
  <pre><code>public class MyPage : ContentPage, IViewFor&lt;MyPageModel&gt; //ReactiveContentPage&lt;MyPageModel&gt;
  {
      Entry input;
      //Entry input2;
      Label displayLabel;
      Slider _slider;
      Button _button;
      ListView _listView;
      public MyPage() : base()
      {
          ViewModel = new MyPageModel();
  
          input = new Entry();
          var input2 = new Entry();
          displayLabel = new Label();
          _slider = new Slider();
          _button = new Button { Text = "Save" };
          _listView = new ListView();
          var cell = new DataTemplate(typeof(TextCell));
          cell.SetBinding(TextCell.TextProperty, "name");
          cell.SetBinding(TextCell.DetailProperty, "location");
          _listView.ItemTemplate = cell;
  
          //_listView.ItemsSource = ViewModel.monkeys;
          Content = new StackLayout
          {
              Children = {
                  new Label { Text = "Hello ContentPage" },
                  input, input2,
                  displayLabel,
                  _slider,
                  _button,
                  _listView
              }
          };
  
          this.Bind(ViewModel, x =&gt; x.username, x =&gt; x.input.Text);
          this.Bind(ViewModel, x =&gt; x.username, x =&gt; input2.Text); //error line 
          this.Bind(ViewModel, x =&gt; x.value, x =&gt; x._slider.Value);
          this.OneWayBind(ViewModel, x =&gt; x.monkeys, x =&gt; x._listView.ItemsSource);
          this.OneWayBind(ViewModel, x =&gt; x.save, x =&gt; x._button.Command);
          this.OneWayBind(ViewModel, x =&gt; x.value, x =&gt; x.displayLabel.Text);
      }
  </code></pre>
  
  <p>The above will not compile. It gives the exception below. When input2 is a property of the class, it runs just fine. Also, OneWayBind will not even compile at all.</p>
  
  <pre><code>System.NotSupportedException was unhandled by user code
    HResult=-2146233067
    Message=Unsupported expression type: 'Constant'
    Source=ReactiveUI
    StackTrace:
         at ReactiveUI.ExpressionMixins.GetExpressionChain(Expression This)
         at ReactiveUI.ReactiveNotifyPropertyChangedMixin.SubscribeToExpressionChain[TSender,TValue](TSender source, Expression expression, Boolean beforeChange, Boolean skipInitial)
         at ReactiveUI.WhenAnyMixin.WhenAnyDynamic[TSender,TRet](TSender This, Expression property1, Func`2 selector)
         at ReactiveUI.PropertyBinderImplementation.Bind[TViewModel,TView,TVMProp,TVProp,TDontCare](TViewModel viewModel, TView view, Expression`1 vmProperty, Expression`1 viewProperty, IObservable`1 signalViewUpdate, Object conversionHint, IBindingTypeConverter vmToViewConverterOverride, IBindingTypeConverter viewToVMConverterOverride)
         at ReactiveUI.BindingMixins.Bind[TViewModel,TView,TVMProp,TVProp](TView view, TViewModel viewModel, Expression`1 vmProperty, Expression`1 viewProperty, Object conversionHint, IBindingTypeConverter vmToViewConverterOverride, IBindingTypeConverter viewToVMConverterOverride)
         at App5.MyPage..ctor()
         at App5.App..ctor()
         at App5.WinPhone.MainPage..ctor()
    InnerException:
  </code></pre>
  
  <ul>
  <li>Is this by design or am I doing something wrong here?</li>
  <li>Though it does not run, why does that code compile with <code>Bind</code> but not <code>OneWayBind</code>?</li>
  </ul>