Add noConflict entry mode to @babel/polyfill + change error to warning (#6371)

* feat: add noConflict polyfill entry
* use a warning

packages/babel-polyfill/noConflict.js

@@ -0,0 +1 @@
+require("./lib/noConflict");

packages/babel-polyfill/src/index.js

@@ -1,7 +1,13 @@
-if (global._babelPolyfill) {
-  throw new Error("only one instance of @babel/polyfill is allowed");
-}
-global._babelPolyfill = true;
-
 import "core-js/shim";
 import "regenerator-runtime/runtime";
+
+if (global._babelPolyfill && typeof console !== "undefined" && console.warn) {
+  console.warn(
+    "@babel/polyfill is loaded more than once on this page. This is probably not desirable/intended " +
+      "and may have consequences if different versions of the polyfills are applied sequentially. " +
+      "If you do need to load the polyfill more than once, use @babel/polyfill/noConflict " +
+      "instead to bypass the warning.",
+  );
+}
+
+global._babelPolyfill = true;

packages/babel-polyfill/src/noConflict.js

@@ -0,0 +1,2 @@
+import "core-js/shim";
+import "regenerator-runtime/runtime";