async cleanup() {
    if (!this._cleanupStarted) {
      this._cleanupStarted = true;
      let ui = this.project.ui;
      ui.startProgress('cleaning up');
      ui.writeLine('cleaning up...');

      // ensure any addon treeFor caches are reset
      _resetTreeCache();

      this._onProcessInterrupt.removeHandler(this._cleanup);

      let node = heimdall.start({ name: 'Builder Cleanup' });
      try {
        await this.builder.cleanup();
      } catch (error) {
        ui.writeLine(chalk.red('Cleanup error.'));
        ui.writeError(error);
      } finally {
        ui.stopProgress();
        node.stop();
      }
    }
  }
Delegates to the `cleanup` method of the wrapped Broccoli builder.

@private
@method cleanup
@return {Promise}