Fix checked mode failure in _asyncLoadError

runtime/bin/builtin.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.
 
 library builtin;
 import 'dart:async';
 import 'dart:convert';
 import 'dart:isolate';
 
 
@@ skipping 333 matching lines @@
 
 void _asyncLoadError(tag, uri, libraryUri, error) {
   if (_logBuiltin) {
     _print("_asyncLoadError($uri), error: $error");
   }
   if (tag == Dart_kImportTag) {
     // When importing a library, the libraryUri is the imported
     // uri.
     libraryUri = uri;
   }
-  _asyncLoadErrorCallback(uri, libraryUri, new LoadError(error));
+  _asyncLoadErrorCallback(uri, libraryUri, new LoadError(error.toString()));
   _finishedOneLoadRequest(uri);
 }
 
 
 _loadDataAsyncLoadPort(int tag,
                        String uri,
                        String libraryUri,
                        Uri resourceUri) {
   var receivePort = new ReceivePort();
   receivePort.first.then((dataOrError) {
@@ skipping 101 matching lines @@
   } else {
     name = userUri.substring(index + 1);
     path = userUri.substring(0, index + 1);
   }
 
   path = _filePathFromUri(path);
   var filename = _platformExtensionFileName(name);
 
   return [path, filename, name];
 }