Create closed world for hello world using .dill file

pkg/compiler/lib/src/library_loader.dart

Index: pkg/compiler/lib/src/library_loader.dart
diff --git a/pkg/compiler/lib/src/library_loader.dart b/pkg/compiler/lib/src/library_loader.dart
index 3643b98e0c56550f6ee22ff45eca9a2df5188d3a..52cc0dbea9a1361a9abf5ebb5ded761511bf7ab2 100644
--- a/pkg/compiler/lib/src/library_loader.dart
+++ b/pkg/compiler/lib/src/library_loader.dart
@@ -835,7 +835,8 @@ class DillLibraryLoaderTask extends CompilerTask implements LibraryLoaderTask {
   // away.
   Future<LoadedLibraries> loadLibrary(Uri resolvedUri,
       {bool skipFileWithPartOfTag: false}) {
-    assert(resolvedUri.pathSegments.last.endsWith('.dill'));
+    assert(resolvedUri.pathSegments.last.endsWith('.dill'),
+        'Invalid uri: $resolvedUri');
     Uri readableUri = uriTranslator.translate(null, resolvedUri, null);
     return measure(() async {
       Script script = await scriptLoader.readScript(readableUri, null);