- Avoid trying to write to final fields.

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;
 // NOTE: Do not import 'dart:io' in builtin.
 import 'dart:collection';
 import 'dart:isolate';
 import 'dart:typed_data';
 
@@ skipping 320 matching lines @@
   // the number of requests remains out of sync.
   _loadScriptCallback(req._tag, req._uri, req._libraryUri, data);
   _finishLoadRequest(req);
 }
 
 
 void _asyncLoadError(_LoadRequest req, _LoadError error) {
   if (_traceLoading) {
     _print("_asyncLoadError(${req._uri}), error: $error");
   }
+  var libraryUri = req._libraryUri;
   if (req._tag == Dart_kImportTag) {
     // When importing a library, the libraryUri is the imported
     // uri.
-    req._libraryUri = req._uri;
+    libraryUri = req._uri;
   }
-  _asyncLoadErrorCallback(req._uri, req._libraryUri, error);
+  _asyncLoadErrorCallback(req._uri, libraryUri, error);
   _finishLoadRequest(req);
 }
 
 
 _loadDataFromLoadPort(int tag,
                       String uri,
                       String libraryUri,
                       Uri resourceUri) {
   try {
     _startLoadRequest(tag, uri, libraryUri, resourceUri);
@@ skipping 148 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];
 }