[WIP] Hot reloading (only for Hooks)

Created by: gaearon

I don't know if this is going to work but I'm going to try another time. Hooks were another piece of the puzzle. Maybe it'll work out.

Preserving state is Hooks-only. It's too brittle for classes. With Hooks it "just works" by design.

packages/react-scripts/package.json

@@ -73,8 +73,8 @@
     "workbox-webpack-plugin": "3.6.3"
   },
   "devDependencies": {
-    "react": "^16.3.2",
-    "react-dom": "^16.3.2"
+    "react": "^16.7.0-alpha.2",
+    "react-dom": "^16.7.0-alpha.2"
   },
   "optionalDependencies": {
     "fsevents": "1.2.4"