Insert implicit downcasts in kernel strong mode.

pkg/kernel/lib/repository.dart

 // Copyright (c) 2016, 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 kernel.repository;
 
 import 'ast.dart';
 
 /// Keeps track of which [Library] objects have been created for a given URI.
 ///
 /// To load different files into the same IR, pass in the same repository
 /// object to the loaders.
 class Repository {
   final Map<Uri, Library> _uriToLibrary = <Uri, Library>{};
   final List<Library> libraries = <Library>[];
 
   Library getLibraryReference(Uri uri) {
     assert(uri.hasScheme);
     return _uriToLibrary.putIfAbsent(uri, () => _buildLibraryReference(uri));
   }
 
   Library _buildLibraryReference(Uri uri) {
     assert(uri.hasScheme);
-    var library = new Library(uri, isExternal: true);
+    var library = new Library(uri, isExternal: true)..fileUri = '$uri';
     libraries.add(library);
     return library;
   }
 }