polyfills: fails ont custom relative path to polyfills/dataset

app/javascript/shared/polyfills.js

@@ -1,4 +1,3 @@
-import './polyfills/dataset';
 import 'core-js/stable';
 import 'regenerator-runtime/runtime';
 import 'dom4';
@@ -48,3 +47,79 @@ function polyfillIsConnected(proto) {
 if (!('isConnected' in Node.prototype)) {
   polyfillIsConnected(Node.prototype);
 }
+
+/*
+  @preserve dataset polyfill for IE < 11. See https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/dataset and http://caniuse.com/#search=dataset
+
+  @author ShirtlessKirk copyright 2015
+  @license WTFPL (http://www.wtfpl.net/txt/copying)
+*/
+
+const dash = /-([a-z])/gi;
+const dataRegEx = /^data-(.+)/;
+const hasEventListener = !!document.addEventListener;
+const test = document.createElement('_');
+const DOMAttrModified = 'DOMAttrModified';
+
+let mutationSupport = false;
+
+function clearDataset(event) {
+  delete event.target._datasetCache;
+}
+
+function toCamelCase(string) {
+  return string.replace(dash, function (_, letter) {
+    return letter.toUpperCase();
+  });
+}
+
+function getDataset() {
+  const dataset = {};
+
+  for (let attribute of this.attributes) {
+    let match = attribute.name.match(dataRegEx);
+    if (match) {
+      dataset[toCamelCase(match[1])] = attribute.value;
+    }
+  }
+
+  return dataset;
+}
+
+function mutation() {
+  if (hasEventListener) {
+    test.removeEventListener(DOMAttrModified, mutation, false);
+  } else {
+    test.detachEvent(`on${DOMAttrModified}`, mutation);
+  }
+
+  mutationSupport = true;
+}
+
+if (!test.dataset) {
+  if (hasEventListener) {
+    test.addEventListener(DOMAttrModified, mutation, false);
+  } else {
+    test.attachEvent(`on${DOMAttrModified}`, mutation);
+  }
+
+  // trigger event (if supported)
+  test.setAttribute('foo', 'bar');
+
+  Object.defineProperty(Element.prototype, 'dataset', {
+    get: mutationSupport
+      ? function get() {
+          if (!this._datasetCache) {
+            this._datasetCache = getDataset.call(this);
+          }
+
+          return this._datasetCache;
+        }
+      : getDataset
+  });
+
+  if (mutationSupport && hasEventListener) {
+    // < IE9 supports neither
+    document.addEventListener(DOMAttrModified, clearDataset, false);
+  }
+}

vite.config.ts

@@ -44,7 +44,8 @@ if (shouldBuildLegacy()) {
 export default defineConfig({
   resolve: { alias: { '@utils': '/shared/utils.ts' } },
   build: {
-    sourcemap: true
+    sourcemap: true,
+    polyfillDynamicImport: true
   },
   plugins
 });