Added OneWayToSource binding to BindingDemo.

Now crashing with an NRE.

samples/BindingDemo/MainWindow.xaml

@@ -24,6 +24,7 @@
             <TextBox Watermark="Two Way" UseFloatingWatermark="True" Text="{Binding Path=StringValue}" Name="first"/>
             <TextBox Watermark="One Way" UseFloatingWatermark="True" Text="{Binding Path=StringValue, Mode=OneWay}"/>
             <TextBox Watermark="One Time" UseFloatingWatermark="True" Text="{Binding Path=StringValue, Mode=OneTime}"/>
+            <TextBox Watermark="One Way to Source" UseFloatingWatermark="True" Text="{Binding Path=StringValue, Mode=OneWayToSource}"/>
           </StackPanel>
           <StackPanel Margin="18" Spacing="4" Width="200">
             <TextBlock FontSize="16" Text="Collection Bindings"/>