output.assetFileNames does not work when the project use @vitejs/plugin-legacy

Describe the bug

When the project use vite@2.5.0-beta.3 and @vitejs/plugin-legacy, output.assetFileNames does not work, and the polyfills-legacy file is outside the js directory. But it works well when remove @vitejs/plugin-legacy.

vite.config.js:

import { defineConfig } from 'vite'
import reactRefresh from '@vitejs/plugin-react-refresh'
import legacy from '@vitejs/plugin-legacy'

// https://vitejs.dev/config/
export default defineConfig({
  plugins: [reactRefresh(), legacy()],
  build: {
    rollupOptions: {
      output: {
        entryFileNames: 'assets/js/[name]-[hash].js',
        chunkFileNames: 'assets/js/[name]-[hash].js',
        assetFileNames: assetInfo => {
          if (/\.css$/.test(assetInfo.name)) {
            return 'assets/css/[name]-[hash][extname]'
          }
          return 'assets/img/[name]-[hash][extname]'
        }
      }
    }
  }
})

Because @vitejs/plugin-legacy will create and add an output configuration

packages/plugin-legacy/index.js#L219:

const { rollupOptions } = config.build
const { output } = rollupOptions
if (Array.isArray(output)) {
  rollupOptions.output = [...output.map(createLegacyOutput), ...output]
} else {
  rollupOptions.output = [createLegacyOutput(output), output || {}]
}

When rollupOptions.output is an array, it defaults to ‘<assetsDir>/[name].[hash][extname]’

packages/vite/src/node/plugins/asset.ts#L309:

const output = config.build?.rollupOptions?.output
const assetFileNames =
  (output && !Array.isArray(output) ? output.assetFileNames : undefined) ??
  // defaults to '<assetsDir>/[name].[hash][extname]'
  // slightly different from rollup's one ('assets/[name]-[hash][extname]')
  path.posix.join(config.build.assetsDir, '[name].[hash][extname]')

Reproduction

https://github.com/JR93/vite-asset-filenames-legacy-bug

System Info

System:
    OS: Windows 10 10.0.19041
    CPU: (8) x64 Intel(R) Core(TM) i7-7700 CPU @ 3.60GHz   
    Memory: 8.68 GB / 15.89 GB
  Binaries:
    Node: 12.16.3 - C:\Program Files\nodejs\node.EXE       
    Yarn: 1.22.10 - C:\Program Files\nodejs\yarn.CMD       
    npm: 6.14.4 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Spartan (44.19041.1023.0), Chromium (92.0.902.73)
    Internet Explorer: 11.0.19041.906
  npmPackages:
    vite: ^2.5.0-beta.3 => 2.5.0

Used Package Manager

yarn

Logs

$ vite build
vite v2.5.0 building for production...
✓ 27 modules transformed.
dist/assets/js/index-legacy-21b357f9.js    2.88 KiB / brotli: 1.27 KiB
dist/assets/polyfills-legacy.7969d14b.js   53.90 KiB / brotli: 19.53 KiB
dist/assets/js/vendor-legacy-8de15e13.js   127.96 KiB / brotli: 36.34 KiB
dist/assets/favicon.17e50649.svg     1.49 KiB
dist/assets/logo.ecc203fb.svg        2.61 KiB
dist/index.html                      1.59 KiB
dist/assets/css/index-d93758c6.css   0.77 KiB / brotli: 0.41 KiB
dist/assets/js/index-a1c55ec7.js     1.66 KiB / brotli: 0.68 KiB
dist/assets/js/vendor-9c242a8b.js    127.61 KiB / brotli: 36.05 KiB

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn’t already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it’s a Vue SFC related bug, it should likely be reported to https://github.com/vuejs/vue-next instead.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.