src/fixed-game-loop.js
function() {
    if (this._initialized && this._isPaused) {
      return false;
    }

    this._isPaused = true;
    raf.cancel(this._requestID);

    this._pauseTime = now();
    this._onPause();

    return true;
  }
function() {
    if (this._initialized && this._isPaused) {
      return false;
    }

    this._isPaused = true;
    raf.cancel(this._requestID);

    this._pauseTime = now();
    this._onPause();

    return true;
  }
pauses the timer