code: function getDefaultBuildConfig(appType) {
      if (appType === 'base') {
          throw new Error('illegal app type', appType);
      }
  
      let defaultConfPath = path.join(
          __dirname, '..', 'config', `${appType}.js`
      );
      if (fileUtil.isFileExists(defaultConfPath)) {
          return require(defaultConfPath);
      }
      else {
          return require('../config/base');
      }
  }
docs: 获取默认构建配置文件
  
  @param {string} appType the type of mini application
  @return {Object} get default config from 'lib/config/swan'