add missing import and move isContainer to prototype

Author: frankiefu

polymer-flex-layout/polymer-flex-panel.html

@@ -3,8 +3,15 @@
 Use of this source code is governed by a BSD-style
 license that can be found in the LICENSE file.
 -->
-<polymer-element name="polymer-flex-panel" noscript extends="polymer-flex-layout" isContainer>
+<link rel="import" href="polymer-flex-layout.html">
+
+<polymer-element name="polymer-flex-panel" extends="polymer-flex-layout">
   <template>
     <shadow></shadow>
   </template>
+  <script>
+    Polymer('polymer-flex-panel', {
+      isContainer: true
+    });
+  </script>
 </polymer-element>