onHandleAST(ast, filePath, code) {
    const ev = new PluginEvent({ast});
    ev.data.filePath = filePath;
    ev.data.code = code;
    this._execHandler('onHandleAST', ev);
    return ev.data.ast;
  }
handle AST.
@param {AST} ast - original ast.
@param {string} filePath - source code file path.
@param {string} code - original code.
@returns {AST} handled AST.