With `--modern` build, the `service-worker.js` for the legcy build gets overwritten (when using PWA)

Version

3.11.0

Environment info

  System:
    OS: Windows 7
    CPU: (4) x64 Intel(R) Core(TM) i5-2450M CPU @ 2.50GHz
  Binaries:
    Node: 10.16.3 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.17.3 - C:\Program Files\nodejs\yarn.CMD
    npm: 6.9.0 - C:\Program Files\nodejs\npm.CMD
  npmPackages:
    @fortawesome/vue-fontawesome: ^0.1.6 => 0.1.6
    @types/vue2-hammer:  undefined ()
    @vue/babel-helper-vue-jsx-merge-props:  1.0.0
    @vue/babel-plugin-transform-vue-jsx:  1.0.0
    @vue/babel-preset-app:  3.10.0
    @vue/babel-preset-jsx:  1.1.0
    @vue/babel-sugar-functional-vue:  1.0.0
    @vue/babel-sugar-inject-h:  1.0.0
    @vue/babel-sugar-v-model:  1.0.0
    @vue/babel-sugar-v-on:  1.1.0
    @vue/cli-overlay:  3.10.0
    @vue/cli-plugin-babel: ^3.9.2 => 3.10.0
    @vue/cli-plugin-pwa: ^3.9.0 => 3.10.0
    @vue/cli-plugin-typescript: ^3.9.0 => 3.10.0
    @vue/cli-service: ^3.9.3 => 3.10.0
    @vue/cli-shared-utils:  3.10.0
    @vue/component-compiler-utils:  2.6.0 (3.0.0)
    @vue/preload-webpack-plugin:  1.1.1
    @vue/web-component-wrapper:  1.2.0
    bootstrap-vue: ^2.0.0-rc.27 => 2.0.0-rc.28
    portal-vue:  2.1.6
    svg-to-vue:  0.4.0
    typescript: ^3.5.3 => 3.5.3
    vue: ^2.6.10 => 2.6.10
    vue-class-component: ^7.1.0 => 7.1.0
    vue-cli-webpack:  1.0.0
    vue-functional-data-merge:  3.1.0
    vue-hot-reload-api:  2.3.3
    vue-loader:  15.7.1
    vue-property-decorator: ^8.2.1 => 8.2.1
    vue-router: ^3.0.7 => 3.1.2
    vue-style-loader:  4.1.2
    vue-svg-loader: ^0.12.0 => 0.12.0
    vue-template-compiler: ^2.6.10 => 2.6.10
    vue-template-es2015-compiler:  1.9.1
    vue2-hammer: ^2.1.2 => 2.1.2
  npmGlobalPackages:
    @vue/cli: Not Found

Steps to reproduce

Run vue-cli-service build --modern on any project that uses the PWA plugin (I guess), e.g. https://github.com/ptandler/enneagram-explorer

What is expected?

Actually I'm not sure if there are legacy browsers around that actually support service-workers.

If yes: then the service-worker.js should not be overwritten.

If no, then the service-worker.js should not be generated for the legacy build (or the legacy's precache-manifest should be removed).

What is actually happening?

When looking at the build console output, you see that service-worker.js is generated twice, but the modern version overwrites the legacy version

"C:\Program Files\nodejs\node.exe" "C:\Program Files\nodejs\node_modules\yarn\bin\yarn.js" run build
yarn run v1.17.3
$ vue-cli-service build --modern

\  Building legacy bundle for production...Starting type checking and linting service...
[...]
  File                                      Size             Gzipped

  docs\js\chunk-vendors-legacy.395a3bc0.    519.38 KiB       146.06 KiB
  js
  docs\js\app-legacy.eae3bd5c.js            84.41 KiB        16.89 KiB
**docs\service-worker.js**                    0.95 KiB         0.54 KiB 
  docs\precache-manifest.95c41e38370b2a3    0.37 KiB         0.22 KiB
  083d5f01432fab8b5.js
  docs\css\app.d418b07d.css                 218.38 KiB       30.85 KiB

-  Building modern bundle for production...Starting type checking and linting service...
[...]
  File                                      Size             Gzipped

  docs\js\chunk-vendors.4306b1ac.js         489.21 KiB       134.63 KiB
  docs\js\app.ad45478d.js                   82.37 KiB        16.59 KiB
**docs\service-worker.js**                    0.95 KiB         0.54 KiB
  docs\precache-manifest.85d7558efdc80c5    0.53 KiB         0.27 KiB
  449958e219a5e5d50.js
  docs\css\app.d418b07d.css                 218.38 KiB       30.85 KiB

 DONE  Build complete. The docs directory is ready to be deployed.

I noticed that there were always two versions of the precache-manifest generated I first I thought that the old one just doesn't get deleted.

vuejs / vue-cli