anchor.diff:   * in the webview's "isolated world"; the proxy receives requests in its
    * on-message listener and responds back via chrome.runtime.sendMessage(),
    * received by the Dart side in a chrome.runtime.onMessage listener.
  - * 
  + *
    * The webview with Polymer Designer can be started and terminated multiple
    * times.
  - * 
  + *
    * The primary useful functionality to a client is [setCode] to intialize
    * the Designer with some existing design and [getCode] to extract a new design
    * back after the user has modified it.
  
positive.code:     });
    }
  
    /** 
     * Extracts and returns to the caller the current design code from the
     * Polymer Designer.
     */
  
negative.code:   }
  
    /**
     * Effectively injects and runs a given script in the [_webview]'s scripting
     * context ("main world"). This is achieved by creating a new <script> tag in
     * the [_webview]'s DOM and setting its inner HTML to the script's code.
     * Doing that will cause the script to run and be able to register event
  