Fix Flow (2/2)

Define missing "beforeEach" so that Flow does not complain.

Facebook is probably having these defined in another manner which
makes their internal test suite pass.

local-cli/__tests__/fs-mock-test.js

@@ -17,6 +17,7 @@
 
 declare var jest: any;
 declare var describe: any;
+declare var beforeEach: any;
 declare var expect: any;
 declare var it: any;