code: function(result, onKill) {
    /** @return {boolean} Whether this command is still running. */
    this.isRunning = function() {
      return result.isPending();
    };
  
    /**
     * @return {!promise.Promise.<!Result>} A promise for the result of this
     *     command.
     */
    this.result = function() {
      return result;
    };
  
    /**
     * Sends a signal to the underlying process.
     * @param {string=} opt_signal The signal to send; defaults to
     *     {@code SIGTERM}.
     */
    this.kill = function(opt_signal) {
      onKill(opt_signal || 'SIGTERM');
    };
  }
docs: Represents a command running in a sub-process.
  @param {!promise.Promise.<!Result>} result The command result.
  @constructor