vite 2.6.x + @vitejs/plugin-legacy broken behavior with esbuild minify default

[tjk]

Describe the bug

Please see repro -- should be well explained there. It looks like the legacy polyfill chunks use terser for minification in all cases (https://github.com/vitejs/vite/blob/main/packages/plugin-legacy/index.js#L542) but I'm not sure if it still requires rest of files to NOT use esbuild? (bails in configResolved: https://github.com/vitejs/vite/blob/main/packages/plugin-legacy/index.js#L101)

Hopefully can still benefit from esbuild minification, but if not -- should update documentation and perhaps label a breaking change somewhere?

Reproduction

https://github.com/tjk/vite-plugin-legacy-esbuild-issue

System Info

System:
    OS: Linux 5.10 Ubuntu 20.04.2 LTS (Focal Fossa)
    CPU: (16) x64 11th Gen Intel(R) Core(TM) i9-11900KF @ 3.50GHz
    Memory: 1.94 GB / 12.38 GB
    Container: Yes
    Shell: 5.0.17 - /bin/bash
  Binaries:
    Node: 14.17.0 - ~/.asdf/installs/nodejs/14.17.0/bin/node
    npm: 6.14.13 - ~/.asdf/installs/nodejs/14.17.0/bin/npm
  npmPackages:
    @vitejs/plugin-vue: latest => 1.9.2
    vite: latest => 2.6.2

Used Package Manager

pnpm

Logs

error during build:
Error: Can't use esbuild as the minifier when targeting legacy browsers because esbuild minification is not legacy safe.
    at Object.configResolved (/home/tjk/src/github.com/tjk/vite-plugin-legacy-esbuild-issue/node_modules/.pnpm/@vitejs+plugin-legacy@1.6.0_vite@2.6.2/node_modules/@vitejs/plugin-legacy/index.js:101:15)
    at /home/tjk/src/github.com/tjk/vite-plugin-legacy-esbuild-issue/node_modules/.pnpm/vite@2.6.2_fa61c471ed365d086e71b610b15aabb9/node_modules/vite/dist/node/chunks/dep-24231913.js:76446:127
    at Array.map (<anonymous>)
    at resolveConfig (/home/tjk/src/github.com/tjk/vite-plugin-legacy-esbuild-issue/node_modules/.pnpm/vite@2.6.2_fa61c471ed365d086e71b610b15aabb9/node_modules/vite/dist/node/chunks/dep-24231913.js:76446:35)
    at processTicksAndRejections (internal/process/task_queues.js:95:5)
    at async doBuild (/home/tjk/src/github.com/tjk/vite-plugin-legacy-esbuild-issue/node_modules/.pnpm/vite@2.6.2_fa61c471ed365d086e71b610b15aabb9/node_modules/vite/dist/node/chunks/dep-24231913.js:51168:20)
    at async build (/home/tjk/src/github.com/tjk/vite-plugin-legacy-esbuild-issue/node_modules/.pnpm/vite@2.6.2_fa61c471ed365d086e71b610b15aabb9/node_modules/vite/dist/node/chunks/dep-24231913.js:51156:16)
    at async CAC.<anonymous> (/home/tjk/src/github.com/tjk/vite-plugin-legacy-esbuild-issue/node_modules/.pnpm/vite@2.6.2_fa61c471ed365d086e71b610b15aabb9/node_modules/vite/dist/node/cli.js:740:9)

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.