Add polka example

Serves as a generic express middleware-compatible server example. (related to jaredpalmer/after.js#77)

Author: William Esz

examples/with-polka/.gitignore

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+logs
+*.log
+npm-debug.log*
+.DS_Store
+
+coverage
+node_modules
+build
+public/static
+.env.*.local

examples/with-polka/README.md

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+# Razzle Polka Example
+
+## How to use
+
+Download the example [or clone the whole project](https://github.com/jaredpalmer/razzle.git):
+
+```bash
+curl https://codeload.github.com/jaredpalmer/razzle/tar.gz/master | tar -xz --strip=2 razzle-master/examples/with-polka
+cd with-polka
+```
+
+Install it and run:
+
+```bash
+yarn install
+yarn start
+```
+
+## Idea behind the example
+
+An example of how to use a custom, express middleware-compatible server like [polka](https://github.com/lukeed/polka) with razzle. It satisfies the entry points
+`src/index.js` for the server and and `src/client.js` for the browser. HMR works for server-side changes too by creating new instances of the polka server handler.

examples/with-polka/package.json

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+{
+  "name": "razzle-examples-with-polka",
+  "version": "0.8.12",
+  "license": "MIT",
+  "scripts": {
+    "start": "razzle start",
+    "build": "razzle build",
+    "test": "razzle test --env=jsdom",
+    "start:prod": "NODE_ENV=production node build/server.js"
+  },
+  "dependencies": {
+    "polka": "^0.3.4",
+    "react": "^16.2.0",
+    "react-dom": "^16.2.0",
+    "serve-static": "^1.13.2"
+  },
+  "devDependencies": {
+    "razzle": "^0.8.12"
+  }
+}

examples/with-polka/public/favicon.ico

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+User-agent: *
+

examples/with-polka/public/robots.txt

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+body {
+  margin: 0;
+  padding: 0;
+  font-family: -apple-system,
+    BlinkMacSystemFont,
+    "Segoe UI",
+    Helvetica,
+    Arial,
+    sans-serif,
+    "Apple Color Emoji",
+    "Segoe UI Emoji",
+    "Segoe UI Symbol";
+}

examples/with-polka/src/App.css

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+import './App.css';
+
+import React from 'react';
+const App = () => <div>Welcome to Razzle.</div>;
+
+export default App;

examples/with-polka/src/App.js

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+import App from './App';
+import React from 'react';
+import ReactDOM from 'react-dom';
+
+describe('<App />', () => {
+  test('renders without exploding', () => {
+    const div = document.createElement('div');
+    ReactDOM.render(<App />, div);
+  });
+});

examples/with-polka/src/App.test.js

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+import React from 'react';
+import { hydrate } from 'react-dom';
+import App from './App';
+
+hydrate(<App />, document.getElementById('root'));
+
+if (module.hot) {
+  module.hot.accept();
+}

examples/with-polka/src/client.js

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+import app from './server';
+import http from 'http';
+
+const server = http.createServer(app);
+
+let currentApp = app;
+
+server.listen(process.env.PORT || 3000, error => {
+  if (error) {
+    console.log(error);
+  }
+
+  console.log('🚀 started');
+});
+
+if (module.hot) {
+  console.log('✅  Server-side HMR Enabled!');
+
+  module.hot.accept('./server', () => {
+    console.log('🔁  HMR Reloading `./server`...');
+    server.removeListener('request', currentApp);
+    const newApp = require('./server').default;
+    server.on('request', newApp);
+    currentApp = newApp;
+  });
+}