fix(plugin-legacy): bake-in Promise polyfill, fix #4414 (#4440)

Author: AlexandreBonaventure

packages/plugin-legacy/index.js

@@ -44,7 +44,12 @@ function viteLegacyPlugin(options = {}) {
   const facadeToLegacyPolyfillMap = new Map()
   const facadeToModernPolyfillMap = new Map()
   const modernPolyfills = new Set()
-  const legacyPolyfills = new Set()
+  // System JS relies on the Promise interface. It needs to be polyfilled for IE 11. (array.iterator is mandatory for supporting Promise.all)
+  const DEFAULT_LEGACY_POLYFILL = [
+    'core-js/modules/es.promise',
+    'core-js/modules/es.array.iterator'
+  ]
+  const legacyPolyfills = new Set(DEFAULT_LEGACY_POLYFILL)
 
   if (Array.isArray(options.modernPolyfills)) {
     options.modernPolyfills.forEach((i) => {