Remove the workaround for issue 19815 in http_multi_server.

pkg/http_multi_server/lib/src/utils.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 http_multi_server.utils;
 
 import 'dart:async';
 import 'dart:io';
 
-// TODO(nweiz): Revert this to the version of [mergeStreams] found elsewhere in
-// the repo once issue 19815 is fixed in dart:io.
 /// Merges all streams in [streams] into a single stream that emits all of their
 /// values.
 ///
 /// The returned stream will be closed only when every stream in [streams] is
 /// closed.
 Stream mergeStreams(Iterable<Stream> streams) {
   var subscriptions = new Set();
   var controller;
   controller = new StreamController(onListen: () {
     for (var stream in streams) {
       var subscription;
-      subscription = stream.listen(controller.add, onError: (error, trace) {
-        if (subscriptions.length == 1) {
-          // If the last subscription errored, pass it on.
-          controller.addError(error, trace);
-        } else {
-          // If only one of the subscriptions has an error (usually IPv6 failing
-          // late), then just remove that subscription and ignore the error.
-          subscriptions.remove(subscription);
-          subscription.cancel();
-        }
-      }, onDone: () {
+      subscription = stream.listen(controller.add,
+          onError: controller.addError,
+          onDone: () {
         subscriptions.remove(subscription);
         if (subscriptions.isEmpty) controller.close();
       });
       subscriptions.add(subscription);
     }
   }, onCancel: () {
     for (var subscription in subscriptions) {
       subscription.cancel();
     }
   }, onPause: () {
     for (var subscription in subscriptions) {
       subscription.pause();
     }
   }, onResume: () {
     for (var subscription in subscriptions) {
       subscription.resume();
     }
   }, sync: true);
 
   return controller.stream;
 }
+
+/// A cache for [supportsIpV6].
+bool _supportsIpV6;
+
+/// Returns whether this computer supports binding to IPv6 addresses.
+Future<bool> get supportsIpV6 {
+  if (_supportsIpV6 != null) return new Future.value(_supportsIpV6);
+
+  return ServerSocket.bind(InternetAddress.LOOPBACK_IP_V6, 0).then((socket) {
+    _supportsIpV6 = true;
+    socket.close();
+    return true;
+  }).catchError((error) {
+    if (error is! SocketException) throw error;
+    _supportsIpV6 = false;
+    return false;
+  });
+}