anchor.diff:  @CustomTag('cde-polymer-designer')
   class CdePolymerDesigner extends PolymerElement {
     js.JsObject _webview;
  -  Completer _webviewReady = new Completer();
  +  Completer _webviewReady;
     Completer<String> _codeExported;
   
     CdePolymerDesigner.created() : super.created();
  
positive.code:   void attached() {
      super.attached();
  
      _webview = new js.JsObject.fromBrowserObject($['webview']);
      _webview.callMethod('addEventListener', ['contentload', _onContentLoad]);
    }
  
    Future reload() {
      final completer = new Completer();
      _webview.callMethod('reload', [(_) => completer.complete()]);
      return completer.future;
  
negative.code:     });
    }
  
    Future _injectDesignerProxy() {
      // TODO(ussuri): Check the sender?
      return _injectScriptInWebviewMainWorld(r'''
          window.addEventListener("message", function(event) {
            var designer = document.querySelector("#designer");
            var action = event.data['action'];
  