code: function createExecutor(serverUrl) {
    let client = serverUrl.then(url => new http.HttpClient(url));
    let executor = new http.Executor(client);
    configureExecutor(executor);
    return executor;
  }
docs: Creates a command executor with support for Marionette's custom commands.
  @param {!Promise<string>} serverUrl The server's URL.
  @return {!command.Executor} The new command executor.