[now dev] Add support for proxying to builder-specific dev servers (#4089)

This is an extension to the Runtime API, where a runtime can optionally define a `startDevServer()` function which is responsible for spawning a single-serve dev server for an individual HTTP request (the dev server is booted up upon receiving an HTTP request, and gets shut down by `now dev` after the HTTP request is completed). For runtimes that define this function, the `build()` function will never be executed, which avoids a lot of unnecessary processing for a dev environment.

Some things this accomplishes:

 * Retains the proper stack trace for errors.
 * Ensures that if a source code file is changed, and then an HTTP request is sent, it's guaranteed to be using the latest code (no file watching, or re-compilations).
 * Avoids creating a Lambda zip file (just to immediately unpack it for dev).
 * Avoids `@zeit/fun` completely, which loses some "correctness" (i.e. function is not frozen in between requests).
 * Backwards compatible with older Now CLIs - versions that don't know about `startDevServer()` will just invoke `build()`, and there's no version change required in the Runtime.

packages/now-node/src/dev-server.ts

@@ -0,0 +1,54 @@
+import http from 'http';
+import path from 'path';
+import { createServerWithHelpers } from './helpers';
+
+function listen(
+  server: http.Server,
+  port: number,
+  host: string
+): Promise<void> {
+  return new Promise(resolve => {
+    server.listen(port, host, () => {
+      resolve();
+    });
+  });
+}
+
+async function main() {
+  const entrypoint = process.env.NOW_DEV_ENTRYPOINT;
+  if (!entrypoint) {
+    throw new Error('`NOW_DEV_ENTRYPOINT` must be defined');
+  }
+
+  //const shouldAddHelpers = true;
+
+  const entrypointPath = path.join(process.cwd(), entrypoint);
+  const handler = await import(entrypointPath);
+
+  /*
+  const server = http.createServer((req, res) => {
+    Promise.resolve(true).then(() => handler.default(req, res)).catch(err => {
+      console.error('Caught error from HTTP handler:', err);
+      if (!res.headersSent) {
+        res.statusCode = 500;
+        res.end('Internal server error\n');
+      }
+    });
+  });
+  */
+  const server = createServerWithHelpers(handler.default);
+
+  await listen(server, 0, '127.0.0.1');
+
+  const address = server.address();
+  if (typeof process.send === 'function') {
+    process.send(address);
+  } else {
+    console.log('Dev server listening:', address);
+  }
+}
+
+main().catch(err => {
+  console.error(err);
+  process.exit(1);
+});