fix #136, support watch exprs and two way bindings for component fields

example/explainer/countcomponent5.html

Index: example/explainer/countcomponent5.html
diff --git a/example/explainer/countcomponent5.html b/example/explainer/countcomponent5.html
index e63ba1a051b53d4cae9400f6038ea542521d9064..526d2d276e5e38806abff168a517c101499897e5 100644
--- a/example/explainer/countcomponent5.html
+++ b/example/explainer/countcomponent5.html
@@ -29,8 +29,8 @@ BSD-style license that can be found in the LICENSE file.
     </script>
   </element>
   <div class="well">
-    <div is="x-click-counter" data-value="count: myNumber"></div>
-    <div is="x-click-counter" data-value="count: 5"></div>
+    <div is="x-click-counter" count="{{myNumber}}"></div>
+    <div is="x-click-counter" count="{{5}}"></div>
   </div>
   <script type="application/dart">
     int myNumber = 12;