Make global variables non-enumerable

src/polyfill.ts

@@ -14,7 +14,6 @@ import {
   WritableStreamDefaultWriter
 } from './ponyfill';
 import { globals } from './utils';
-import ObjectAssign from './stub/object-assign';
 
 // Export
 export * from './ponyfill';
@@ -40,5 +39,13 @@ const exports = {
 
 // Add classes to global scope
 if (typeof globals !== 'undefined') {
-  ObjectAssign(globals, exports);
+  for (const prop in exports) {
+    if (Object.prototype.hasOwnProperty.call(exports, prop)) {
+      Object.defineProperty(globals, prop, {
+        value: exports[prop as (keyof typeof exports)],
+        writable: true,
+        configurable: true
+      });
+    }
+  }
 }