code: function readString(walker) {
      var startCode = walker.currentCode();
      var startIndex = walker.index;
      var charCode;
  
      walkLoop: while ((charCode = walker.nextCode())) {
          switch (charCode) {
              case 92: // \
                  walker.go(1);
                  break;
              case startCode:
                  walker.go(1);
                  break walkLoop;
          }
      }
  
      var literal = walker.cut(startIndex, walker.index);
      return {
          type: ExprType.STRING,
          // 处理字符转义
          value: (new Function('return ' + literal))()
      };
  }
docs: 读取字符串
  
  @param {Walker} walker 源码读取对象
  @return {Object}