Fix checked mode error

runtime/lib/lib_prefix.dart

 // Copyright (c) 2014, 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:async";
 import "dart:isolate";
 
 // This type corresponds to the VM-internal class LibraryPrefix.
 class _LibraryPrefix {
   bool _load() native "LibraryPrefix_load";
-  Error _loadError() native "LibraryPrefix_loadError";
+  Object _loadError() native "LibraryPrefix_loadError";
   bool isLoaded() native "LibraryPrefix_isLoaded";
   bool _invalidateDependentCode()
       native "LibraryPrefix_invalidateDependentCode";
 
   loadLibrary() {
     for (int i = 0; i < _outstandingLoadRequests.length; i++) {
       if (_outstandingLoadRequests[i][0] == this) {
         return _outstandingLoadRequests[i][1].future;
       }
     }
@@ skipping 49 matching lines @@
       completer.completeError(error);
     } else if (prefix._load()) {
       prefix._invalidateDependentCode();
       completer.complete(true);
     } else {
       stillOutstandingLoadRequests.add(outstandingRequests[i]);
     }
   }
   _outstandingLoadRequests = stillOutstandingLoadRequests;
 }