Insert implicit downcasts in kernel strong mode.

pkg/kernel/lib/repository.dart

Index: pkg/kernel/lib/repository.dart
diff --git a/pkg/kernel/lib/repository.dart b/pkg/kernel/lib/repository.dart
index 265a75c25e39b9db88a3a4de2c22758bf2a3a329..73cc1a9ac74bcac1274b5242ffe74a176fe6baf7 100644
--- a/pkg/kernel/lib/repository.dart
+++ b/pkg/kernel/lib/repository.dart
@@ -20,7 +20,7 @@ class Repository {
 
   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;
   }