code: function compile(file, options) {
      let {envConfigKey} = options;
      let obj = JSON.parse(file.content.toString());
      let result = {};
      Object.keys(obj).forEach(k => {
          if (!OKAM_ENV_KEY_REGEXP.test(k)) {
              result[k] = obj[k];
          }
      });
  
      let appSpecConf = obj[envConfigKey];
      let isAppConfig = file.isAppConfig;
      appSpecConf && Object.keys(appSpecConf).forEach(k => {
          let value = appSpecConf[k];
          if (value == null) {
              // remove null property
              if (result.hasOwnProperty(k)) {
                  delete result[k];
              }
          }
          else if (isAppConfig && k === 'window') {
              let windowConf = Object.assign({}, result[k], value);
              result[k] = filterNullPropertyConfig(windowConf);
          }
          else {
              result[k] = value;
          }
      });
  
      return {
          content: JSON.stringify(result, null, 4)
      };
  }
docs: Compile config json: remove not current app type config info
  merge the common config with the specified app config.
  
  @param {Object} file the file to process
  @param {Object} options the compile options
  @return {{content: string}}