Add missing file

dom-shim.d.ts

@@ -0,0 +1,10 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+// d.ts shims provide types for things we use internally but are not part
+// of this package's surface area.
+
+interface Request {}
+interface RequestInit {}
+interface Response {}
+interface Headers {}