One more error code to check.

utils/pub/io.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Helper functionality to make working with IO easier.
  */
 #library('io');
 
 #import('dart:io');
@@ skipping 308 matching lines @@
   // #3657 is fixed.
   uri = _getUri(uri);
 
   var completer = new Completer<String>();
   var client = new HttpClient();
   var connection = client.getUrl(uri);
 
   connection.onError = (e) {
     // Show a friendly error if the URL couldn't be resolved.
     if (e is SocketIOException &&
-        (e.osError.errorCode == 8 || e.osError.errorCode == -2)) {
+        (e.osError.errorCode == 8 ||
+         e.osError.errorCode == -2 ||
+         e.osError.errorCode == -5)) {
       e = 'Could not resolve URL "${uri.origin}".';
     }
 
     client.shutdown();
     completer.completeException(e);
   };
 
   connection.onResponse = (response) {
     if (response.statusCode >= 400) {
       client.shutdown();
@@ skipping 209 matching lines @@
   return new Directory(entry);
 }
 
 /**
  * Gets a [Uri] for [uri], which can either already be one, or be a [String].
  */
 Uri _getUri(uri) {
   if (uri is Uri) return uri;
   return new Uri.fromString(uri);
 }