Enable strict mode

Summary: BOOM 💥

Reviewed By: danielbuechele

Differential Revision: D17314462

fbshipit-source-id: 19d848532e1809cdfdd630f91d034bbbc9f6025d

tsconfig.json

@@ -14,7 +14,8 @@
     "esModuleInterop": true,
     "paths": {
       "flipper": ["./src/index.tsx"]
-    }
+    },
+    "strict": true
   },
   "include": ["src/**/*", "types/*", "headless/*"],
   "exclude": ["node_modules", "**/*.spec.ts"]