anchor.diff:     * 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:
  +   * The target `main` must be a subtype of one of these three signatures:
      *
      * * `main()`
      * * `main(args)`
  
positive.code:    * 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.
     */
    external static Future<Isolate> spawn(void entryPoint(message), var message);
  
  
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);
  