Don't propagate http client error, when no response object is available.

tests/standalone/src/io/HttpClientTest.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.
 
 #import("dart:io");
 #import("dart:uri");
+#import("dart:isolate");
 
 void testGoogle() {
   HttpClient client = new HttpClient();
   var conn = client.get('www.google.com', 80, '/');
 
   conn.onRequest = (HttpClientRequest request) {
     request.keepAlive = false;
     request.outputStream.close();
   };
   conn.onResponse = (HttpClientResponse response) {
@@ skipping 38 matching lines @@
 }
 
 void testInvalidUrl() {
   HttpClient client = new HttpClient();
   Expect.throws(
       () => client.getUrl(new Uri.fromString('ftp://www.google.com')));
   Expect.throws(
       () => client.getUrl(new Uri.fromString('http://usr:pwd@www.google.com')));
 }
 
+void testBadHostName() {
+  HttpClient client = new HttpClient();
+  HttpClientConnection connection =
+      client.get("some.bad.host.name.7654321", 0, "/");
+  connection.onRequest = (HttpClientRequest request) {
+    Expect.fail("Should not open a request on bad hostname");
+  };
+  ReceivePort port = new ReceivePort();
+  connection.onError = (Exception error) {
+    port.close();  // We expect onError to be called, due to bad host name.
+  };
+}
+
 void main() {
   // TODO(sgjesse): Making empty www.google.com requests seems to fail
   //on buildbot.
   //testGoogle();
   //testGoogleUrl();
   testInvalidUrl();
+  testBadHostName();
 }