Refactor Intl polyfills and remove from ES5 entrypoints (#16349)

build-scripts/webpack.cjs

@@ -132,6 +132,17 @@ const createWebpackConfig = ({
         ),
         path.resolve(paths.polymer_dir, "src/util/empty.js")
       ),
+      // See `src/resources/intl-polyfill-legacy.ts` for explanation
+      !latestBuild &&
+        new webpack.NormalModuleReplacementPlugin(
+          new RegExp(
+            path.resolve(paths.polymer_dir, "src/resources/intl-polyfill.ts")
+          ),
+          path.resolve(
+            paths.polymer_dir,
+            "src/resources/intl-polyfill-legacy.ts"
+          )
+        ),
       !isProdBuild && new LogStartCompilePlugin(),
     ].filter(Boolean),
     resolve: {