onESMLeave (code, filePath, ast) {
        let { output } = activeModules[filePath];

        let payload = {
            code: output.toString(),
            map: output.generateMap({ source: filePath })
        };

        delete activeModules[filePath];

        return payload;
    }
@param {string} code 
@param {string} filePath 
@param {ESTree} ast 
@return {{ code: string, map: RollupSourceMap }}