Building for legacy browsers fails

[lovasoa]

Describe the bug

Can a svelte kit app work on browsers without native es modules support ?

I tried using @vitejs/plugin-legacy, but compilation fails with the following error.

Logs

 > @vitejs/plugin-legacy does not support library mode.
Error: @vitejs/plugin-legacy does not support library mode.
    at Object.configResolved (/home/runner/work/sanipasse/sanipasse/node_modules/@vitejs/plugin-legacy/index.js:162:15)
    at /home/runner/work/sanipasse/sanipasse/node_modules/vite/dist/node/chunks/dep-0ed4fbc0.js:71748:127
    at Array.map (<anonymous>)
    at resolveConfig (/home/runner/work/sanipasse/sanipasse/node_modules/vite/dist/node/chunks/dep-0ed4fbc0.js:71748:35)
    at async doBuild (/home/runner/work/sanipasse/sanipasse/node_modules/vite/dist/node/chunks/dep-0ed4fbc0.js:45273:20)
    at async Object.build (/home/runner/work/sanipasse/sanipasse/node_modules/vite/dist/node/chunks/dep-0ed4fbc0.js:45261:16)
    at async build_service_worker (file:///home/runner/work/sanipasse/sanipasse/node_modules/@sveltejs/kit/dist/chunks/index4.js:710:2)
    at async build (file:///home/runner/work/sanipasse/sanipasse/node_modules/@sveltejs/kit/dist/chunks/index4.js:239:3)
    at async file:///home/runner/work/sanipasse/sanipasse/node_modules/@sveltejs/kit/dist/cli.js:873:23

https://github.com/lovasoa/sanipasse/runs/2956707572?check_suite_focus=true

To Reproduce

in svelte config

 import vite_plugin_legacy from '@vitejs/plugin-legacy';

...

const config = {
	preprocess: preprocess(),
	kit: {
		target: '#svelte',
		adapter,
		vite: {
			plugins: [
				vite_plugin_legacy({
					targets: ['ie >= 11'],
					additionalLegacyPolyfills: ['regenerator-runtime/runtime']
				})
			],

Information about your SvelteKit Installation:

Diagnostics

  Binaries:
    Node: 16.1.0 - /usr/local/bin/node
    npm: 7.11.2 - /usr/local/bin/npm

  npmPackages:
    @sveltejs/adapter-node: next => 1.0.0-next.24 
    @sveltejs/adapter-static: next => 1.0.0-next.13 
    @sveltejs/kit: next => 1.0.0-next.113 
    svelte: ^3.38.2 => 3.38.2 

Severity

blocking some users

[Conduitry]

This isn't supported yet, but is on the roadmap post-1.0 - #12.