code: function () {
              if (alias) {
                  return alias;
              }
              var prefix = './';
              var length = dir.split('/').length - 2;
              if (length) {
                  prefix = (new Array(length + 1)).join('../');
              }
              return prefix + runtime.replace(/\.js$/, '');
          }
docs: 计算主入口相对于当前模板路径