Add exception information to help diagnose issue 8865

tests/standalone/io/http_content_length_test.dart

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2013, 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:isolate";
 import "dart:io";
 
 void testNoBody(int totalConnections, bool explicitContentLength) {
   var errors = 0;
   HttpServer.bind("127.0.0.1", 0, totalConnections).then((server) {
     server.listen(
@@ skipping 72 matching lines @@
                 response.addString("x");
                 Expect.throws(() => response.contentLength = 3,
                               (e) => e is HttpException);
                 response.addString("x");
                 response.addString("x");
                 response.done
                     .then((_) {
                       Expect.fail("Unexpected successful response completion");
                     })
                     .catchError((e) {
-                      Expect.isTrue(e.error is HttpException);
+                      Expect.isTrue(e.error is HttpException, "[$e] [${e.error}]");
                     });
                 response.close();
                 Expect.throws(() => response.addString("x"),
                               (e) => e is StateError);
               });
         },
         onError: (e) => Expect.fail("Unexpected error $e"));
 
     int count = 0;
     HttpClient client = new HttpClient();
@@ skipping 128 matching lines @@
 
 void main() {
   testNoBody(5, false);
   testNoBody(5, true);
   testBody(5, false);
   testBody(5, true);
   testBodyChunked(5, false);
   testBodyChunked(5, true);
   testHttp10();
 }