Add the package implementation.

lib/src/async_handler.dart

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:async/async.dart';
+import 'package:shelf/shelf.dart';
+
+class AsyncHandler {
+  final ResultFuture<Handler> _future;
+
+  AsyncHandler(Future<Handler> future) : _future = new ResultFuture(future);
+
+  call(Request request) {
+    if (_future.result == null) {
+      return _future.then((handler) => handler(request));
+    }
+
+    if (_future.result.isError) return _future;
+
+    return _future.result.asValue.value(request);
+  }
+}