Support Request hijacking in Shelf, using a similar API to Rack.

pkg/shelf/lib/src/util.dart

 // Copyright (c) 2014, 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.
 
 library shelf.util;
 
 import 'dart:async';
 
 import 'package:stack_trace/stack_trace.dart';
 
+/// Like [new Future], but avoids around issue 11911 by using [new Future.value]
+/// under the covers.
+Future newFuture(callback()) => new Future.value().then((_) => callback());
+
 /// Like [Future.sync], but wraps the Future in [Chain.track] as well.
 Future syncFuture(callback()) => Chain.track(new Future.sync(callback));
 
 /// Run [callback] and capture any errors that would otherwise be top-leveled.
 ///
 /// If [this] is called in a non-root error zone, it will just run [callback]
 /// and return the result. Otherwise, it will capture any errors using
 /// [runZoned] and pass them to [onError].
 catchTopLevelErrors(callback(), void onError(error, StackTrace stackTrace)) {
   if (Zone.current.inSameErrorZone(Zone.ROOT)) {
     return runZoned(callback, onError: onError);
   } else {
     return callback();
   }
 }
 
 /// Returns a [Map] with the values from [original] and the values from
 /// [updates].
 ///
 /// For keys that are the same between [original] and [updates], the value in
 /// [updates] is used.
 ///
 /// If [updates] is `null` or empty, [original] is returned unchanged.
 Map updateMap(Map original, Map updates) {
   if (updates == null || updates.isEmpty) return original;
 
   return new Map.from(original)
       ..addAll(updates);
 }