完整错误信息：

events.js:173
      throw er; // Unhandled 'error' event
      ^

Error: spawn rollup ENOENT
    at Process.ChildProcess._handle.onexit (internal/child_process.js:248:19)
    at onErrorNT (internal/child_process.js:431:16)
    at processTicksAndRejections (internal/process/task_queues.js:83:17)
    at process.runNextTicks [as _tickCallback] (internal/process/task_queues.js:58:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:828:11)
    at internal/main/run_main_module.js:17:11
Emitted 'error' event at:
    at Process.ChildProcess._handle.onexit (internal/child_process.js:254:12)
    at onErrorNT (internal/child_process.js:431:16)
    [... lines matching original stack trace ...]
    at internal/main/run_main_module.js:17:11

这个错误信息让人完全摸不到头脑啊，我运行的是：

yarn start

所指向的命令是：

rollup -c -w

最后找了半天，原来是package.json中根本没有安装rollup，先安装rollup:

yarn add rollup -D

问题解决。