anchor.diff:     * Usually the initial [message] contains a [SendPort] so
      * that the spawner and spawnee can communicate with each other.
      *
  -   * Returns a future that will complete with an [Isolate] instance. The
  -   * isolate instance can be used to control the spawned isolate.
  +   * Returns a future that will complete with an [Isolate] instance if the
  +   * spawning succeeded. It will complete with an error otherwise.
      */
     external static Future<Isolate> spawn(void entryPoint(message), var message);
   
  
positive.code:    * The isolate starts executing the top-level `main` function of the library
     * with the given URI.
     *
     * The target `main` may have one of three signatures:
     *
     * * `main()`
     * * `main(args)`
  
negative.code:    * When present, the parameter `args` is set to the provided [args] list.
     * When present, the parameter `message` is set to the initial [message].
     *
     * Returns a future that will complete with an [Isolate] instance if the
     * spawning succeeded. It will complete with an error otherwise.
     */
    external static Future<Isolate> spawnUri(
        Uri uri, List<String> args, var message);
  