[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/babel-plugin-hot-reload/tests/__fixtures__/components/Hello.js

+import React from 'react';
+
+export default class extends React.Component {
+  render() {}
+}

packages/babel-plugin-hot-reload/tests/__fixtures__/components/Layout.js

+import React from 'react';
+
+export default class extends React.Component {
+  render() {}
+}

packages/babel-plugin-hot-reload/tests/__fixtures__/assignment.js

+let LazyCC;
+LazyCC = 5;
+
+let DoubleLazyCC = LazyCC;

packages/babel-plugin-hot-reload/tests/__fixtures__/default-export-class.js

+import React from 'react';
+import Hello from './components/Hello';
+
+export default class App extends React.Component {
+  render() {
+    return <Hello />;
+  }
+}

packages/babel-plugin-hot-reload/tests/__fixtures__/default-export.js

+import React from 'react';
+import Hello from './components/Hello';
+import Layout from './components/Layout';
+import './App.css';
+
+export default function App({ children }) {
+  return (
+    <Layout>
+      <Hello />
+    </Layout>
+  );
+}