code: function getAst(code) {
      var ast = null;
  
      try {
          ast = esprima.parse(code, {
              range: true
          });
      }
      catch (e) {
          throw 'can\'t parse amd code';
      }
  
      return ast;
  }
docs: 获取ast树
  @param  {string} code 代码片段
  @return {Object}      ast树