fix(plugin-legacy): turn off babel loose mode (#3406)

Author: nihalgonsalves

packages/playground/legacy/__tests__/legacy.spec.ts

@@ -7,3 +7,8 @@ test('should work', async () => {
 test('import.meta.env.LEGACY', async () => {
   expect(await page.textContent('#env')).toMatch(isBuild ? 'true' : 'false')
 })
+
+// https://github.com/vitejs/vite/issues/3400
+test('transpiles down iterators correctly', async () => {
+  expect(await page.textContent('#iterators')).toMatch('hello')
+})

packages/playground/legacy/index.html

@@ -1,3 +1,4 @@
 <h1 id="app"></h1>
 <div id="env"></div>
+<div id="iterators"></div>
 <script type="module" src="./main.js"></script>

packages/playground/legacy/main.js

@@ -15,3 +15,9 @@ if (import.meta.env.LEGACY) {
 }
 
 document.getElementById('env').textContent = `is legacy: ${isLegacy}`
+
+// Iterators
+
+document.getElementById('iterators').textContent = [...new Set(['hello'])].join(
+  ''
+)

packages/plugin-legacy/index.js

@@ -263,7 +263,7 @@ function viteLegacyPlugin(options = {}) {
               targets,
               modules: false,
               bugfixes: true,
-              loose: true,
+              loose: false,
               useBuiltIns: needPolyfills ? 'usage' : false,
               corejs: needPolyfills
                 ? { version: 3, proposals: false }