Create closed world for hello world using .dill file

pkg/compiler/lib/src/library_loader.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 dart2js.library_loader;
 
 import 'dart:async';
 
 import 'common/names.dart' show Uris;
 import 'common/tasks.dart' show CompilerTask, Measurer;
@@ skipping 817 matching lines @@
       this.reporter, Measurer measurer)
       : _allLoadedLibraries = new List<LibraryEntity>(),
         super(measurer);
 
   /// Loads an entire Kernel [Program] from a file on disk (note, not just a
   /// library, so this name is actually a bit of a misnomer).
   // TODO(efortuna): Rename this once the Element library loader class goes
   // 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);
       ir.Program program = new ir.Program();
       // Hack because the existing file has a terminating 0 and the
       // BinaryBuilder doesn't expect that.
       var bytes = new List<int>.from(script.file.slowUtf8ZeroTerminatedBytes());
       bytes.removeLast();
       new BinaryBuilder(bytes).readProgram(program);
       return measure(() {
@@ skipping 810 matching lines @@
 }
 
 /// API used by the library loader to synchronously scan a library or
 /// compilation unit and ensure that their library tags are computed.
 abstract class ElementScanner {
   void scanLibrary(LibraryElement library);
   void scanUnit(CompilationUnitElement unit);
 }
 
 const _reuseLibrarySubtaskName = "Reuse library";