Add Flow libdefs for global

Summary:
Changelog: [Internal]

Currently, `global` is typed as `any` and any `global` properties
accesses are untyped. This diff add a flow libdefs for the `global`
object as a start point.

Reviewed By: yungsters

Differential Revision: D30000895

fbshipit-source-id: ab6988d01921a3c2a3434b534b2f69083570fb6d

flow/global.js

@@ -0,0 +1,21 @@
+/**
+ * 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.
+ *
+ * @flow strict
+ * @format
+ */
+
+/**
+ * `global` is a object containing all the global variables for React Native.
+ *
+ * NOTE: Consider cross-platform as well as JS environments compatibility
+ * when defining the types here. Consider both presence (`?`) as well as
+ * writeability (`+`) when defining types.
+ */
+declare var global: {
+  // Undeclared properties are implicitly `any`.
+  [string | symbol]: any,
+};