anchor.diff:      ''');
     }
   
  -  Future _injectWebviewScriptingContextScript(String script) {
  +  Future _injectScriptInWebviewMainWorld(String script) {
       script = script.replaceAll('"', r'\"').replaceAll('\n', r'\n');
       return _executeScriptInWebview(r'''
           (function() {
  
positive.code: @CustomTag('cde-polymer-designer')
  class CdePolymerDesigner extends PolymerElement {
    js.JsObject _webview;
    Completer _webviewReady = new Completer();
    Completer<String> _codeExported;
  
    CdePolymerDesigner.created() : super.created();
  
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'];
  