Inject polyfills where used (#18719)

build-scripts/babel-plugins/custom-polyfill-plugin.js

@@ -0,0 +1,56 @@
+import defineProvider from "@babel/helper-define-polyfill-provider";
+
+// List of polyfill keys with supported browser targets for the functionality
+const PolyfillSupport = {
+  fetch: {
+    android: 42,
+    chrome: 42,
+    edge: 14,
+    firefox: 39,
+    ios: 10.3,
+    opera: 29,
+    opera_mobile: 29,
+    safari: 10.1,
+    samsung: 4.0,
+  },
+  proxy: {
+    android: 49,
+    chrome: 49,
+    edge: 12,
+    firefox: 18,
+    ios: 10.0,
+    opera: 36,
+    opera_mobile: 36,
+    safari: 10.0,
+    samsung: 5.0,
+  },
+};
+
+// Map of global variables and/or instance and static properties to the
+// corresponding polyfill key and actual module to import
+const polyfillMap = {
+  global: {
+    Proxy: { key: "proxy", module: "proxy-polyfill" },
+    fetch: { key: "fetch", module: "unfetch/polyfill" },
+  },
+  instance: {},
+  static: {},
+};
+
+// Create plugin using the same factory as for CoreJS
+export default defineProvider(
+  ({ createMetaResolver, debug, shouldInjectPolyfill }) => {
+    const resolvePolyfill = createMetaResolver(polyfillMap);
+    return {
+      name: "HA Custom",
+      polyfills: PolyfillSupport,
+      usageGlobal(meta, utils) {
+        const polyfill = resolvePolyfill(meta);
+        if (polyfill && shouldInjectPolyfill(polyfill.desc.key)) {
+          debug(polyfill.desc.key);
+          utils.injectGlobalImport(polyfill.desc.module);
+        }
+      },
+    };
+  }
+);