Add an http_multi_server package.

pkg/http_multi_server/README.md

Index: pkg/http_multi_server/README.md
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+An implementation of `dart:io`'s [HttpServer][] that wraps multiple servers and
+forwards methods to all of them. It's useful for serving the same application on
+multiple network interfaces while still having a unified way of controlling the
+servers. In particular, it supports serving on both the IPv4 and IPv6 loopback
+addresses using [HttpMultiServer.loopback][].
+
+```dart
+import 'package:http_multi_server/http_multi_server.dart';
+import 'package:shelf/shelf.dart' as shelf;
+import 'package:shelf/shelf_io.dart' as shelf_io;
+
+void main() {
+  // Both http://127.0.0.1:8080 and http://[::1]:8080 will be bound to the same
+  // server.
+  HttpMultiServer.loopback(8080).then((server) {
+    shelf_io.serveRequests(server, (request) {
+      return new shelf.Response.ok("Hello, world!");
+    });
+  });
+}
+```
+
+[HttpServer]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart-io.HttpServer
+
+[HttpMultiServer.loopback]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/http_multi_server/http_multi_server.HttpMultiServer#id_loopback