feat(kernel): add restore() fn to PLATFORM

packages/kernel/src/platform.ts

@@ -281,7 +281,21 @@ const hasOwnProperty = Object.prototype.hasOwnProperty as unknown as {
 const emptyArray = Object.freeze([]) as unknown as any[];
 const emptyObject = Object.freeze({}) as any;
 
-export const PLATFORM = {
+function getRestoreFn(): () => void {
+  const platformKeys = Object.freeze(Object.keys($PLATFORM) as (keyof typeof $PLATFORM)[]);
+
+  function restore(): void {
+    for (const key of platformKeys) {
+      if (PLATFORM[key] !== $PLATFORM[key]) {
+        PLATFORM[key] = $PLATFORM[key];
+      }
+    }
+  }
+
+  return restore;
+}
+
+const $PLATFORM = Object.freeze({
   global: $global,
   emptyArray,
   emptyObject,
@@ -319,5 +333,15 @@ export const PLATFORM = {
   // tslint:disable-next-line:no-any
   setTimeout(handler: ITimerHandler, timeout?: number, ...args: any[]): number {
     return $global.setTimeout(handler, timeout, ...args);
-  }
-};
+  },
+
+  /**
+   * Restore the global `PLATFORM` object to its original state as it was immediately after module initialization.
+   * Useful for when you need to stub out one or more of its methods in a unit test.
+   *
+   * Extraneous properties are NOT removed.
+   */
+  restore: getRestoreFn(),
+});
+
+export const PLATFORM = { ...$PLATFORM };