Building in modern mode with alternative entry leads to failing compiling

[karudo]

Version

3.0.1

Reproduction link

https://github.com/karudo/vue-cli-bug

Node and OS info

Node 10.9.0 / npm 6.2.0 / macos 10.13.6

Steps to reproduce

npm run build

What is expected?

Compiled successfully

What is actually happening?

ERROR Failed to compile with 1 errors 2:18:11 PM

This relative module was not found:

• ./src/main.js in multi ./src/main.js
  ERROR Build failed with errors.

---

If I set alternative entry in modern mode I got fail
vue-cli-service build --modern ./src/main2.js

[haoqunjiang]

For now you can use

vue-cli-service build ./src/main2.js --modern