Remove the workaround for issue 19815 in http_multi_server.

pkg/http_multi_server/lib/http_multi_server.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;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'src/utils.dart';
@@ skipping 81 matching lines @@
             certificateName: certificateName,
             requestClientCertificate: requestClientCertificate));
   }
 
   /// A helper method for initializing loopback servers.
   ///
   /// [bind] should forward to either [HttpServer.bind] or
   /// [HttpServer.bindSecure].
   static Future<HttpServer> _loopback(int port,
       Future<HttpServer> bind(InternetAddress address, int port)) {
-    return bind(InternetAddress.LOOPBACK_IP_V4, port).then((v4Server) {
+    return Future.wait([
+      supportsIpV6,
+      bind(InternetAddress.LOOPBACK_IP_V4, port)
+    ]).then((results) {
+      var supportsIpV6 = results[0];
+      var v4Server = results[1];
+
+      if (!supportsIpV6) return v4Server;
+
       // Reuse the IPv4 server's port so that if [port] is 0, both servers use
       // the same ephemeral port.
       return bind(InternetAddress.LOOPBACK_IP_V6, v4Server.port)
-          .then((v6Server) => new HttpMultiServer([v4Server, v6Server]))
-          .catchError((error) {
-        // If we fail to bind to IPv6, just use IPv4.
-        if (error is SocketException) return v4Server;
-        throw error;
+          .then((v6Server) {
+        return new HttpMultiServer([v4Server, v6Server]);
       });
     });
   }
 
   Future close({bool force: false}) =>
       Future.wait(_servers.map((server) => server.close(force: force)));
 
   /// Returns an HttpConnectionsInfo object summarizing the total number of
   /// current connections handled by all the servers.
   HttpConnectionsInfo connectionsInfo() {
     var info = new HttpConnectionsInfo();
     for (var server in _servers) {
       var subInfo = server.connectionsInfo();
       info.total += subInfo.total;
       info.active += subInfo.active;
       info.idle += subInfo.idle;
       info.closing += subInfo.closing;
     }
     return info;
   }
 }