Add FileSelector shared component

Summary: Add shared FileSelector component compatibe with the new FlipperLib API

Reviewed By: mweststrate

Differential Revision: D32667100

fbshipit-source-id: dca1e8b7693d134a99617e916c7cfd30432cef78

desktop/flipper-plugin/src/utils/assertNever.tsx

@@ -0,0 +1,14 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+export const assertNever: (val: never) => asserts val = (val) => {
+  if (val) {
+    throw new Error(`Assert never failed. Received ${val}`);
+  }
+};