docs: function getDefinePlugin(buildConfig) {
      var definitionsObj = buildConfig.task.js.definitions ? buildConfig.task.js.definitions[buildConfig.env] : {};
  
      var definitions = {};
      for (var key in definitionsObj) {
          definitions[key] = JSON.stringify(definitionsObj[key]);
      }
  
      if (buildConfig.env != 'dev') {
          // 一般约定的优化
          process.env.NODE_ENV = 'production';
          definitions['process.env.NODE_ENV'] = JSON.stringify(process.env.NODE_ENV);
      }
  
      return new webpack.DefinePlugin(definitions);
  }