Fix buildbot on trunk:

dart/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 332 matching lines @@
     native "Builtin_AsyncLoadError";
 
 void _loadScript(int tag, String uri, String libraryUri, List<int> data) {
   // TODO: Currently a compilation error while loading the script is
   // fatal for the isolate. _loadScriptCallback() does not return and
   // the _numOutstandingLoadRequests counter remains out of sync.
   _loadScriptCallback(tag, uri, libraryUri, data);
   _finishedOneLoadRequest(uri);
 }
 
-void _asyncLoadError(int tag, String uri, String libraryUri, LoadError error) {
+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, 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) {
     if (dataOrError is List<int>) {
       _loadScript(tag, uri, libraryUri, dataOrError);
     } else {
-      assert(dataOrError is String);
-      var error = new LoadError(dataOrError.toString());
-      _asyncLoadError(tag, uri, libraryUri, error);
+      _asyncLoadError(tag, uri, libraryUri, dataOrError);
     }
   }).catchError((e) {
-    // Wrap inside a LoadError unless we are already propagating a previously
-    // seen LoadError.
-    var error = (e is LoadError) ? e : new LoadError(e.toString);
-    _asyncLoadError(tag, uri, libraryUri, error);
+    _asyncLoadError(tag, uri, libraryUri, e.toString());
   });
 
   try {
     var msg = [receivePort.sendPort, resourceUri.toString()];
     _loadPort.send(msg);
     _startingOneLoadRequest(uri);
   } catch (e) {
     if (_logBuiltin) {
       _print("Exception when communicating with service isolate: $e");
     }
-    // Wrap inside a LoadError unless we are already propagating a previously
-    // seen LoadError.
-    var error = (e is LoadError) ? e : new LoadError(e.toString);
-    _asyncLoadError(tag, uri, libraryUri, error);
+    _asyncLoadError(tag, uri, libraryUri, e.toString());
     receivePort.close();
   }
 }
 
 // Asynchronously loads script data through a http[s] or file uri.
 _loadDataAsync(int tag, String uri, String libraryUri) {
   if (tag == Dart_kScriptTag) {
     uri = _resolveScriptUri(uri);
   }
 
@@ skipping 73 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];
 }