Clean VM thread shutdown:

runtime/bin/dartutils.h

Index: runtime/bin/dartutils.h
diff --git a/runtime/bin/dartutils.h b/runtime/bin/dartutils.h
index 8a9c9530331c35765c6857e2af72b8e6a1f1a644..83626e0b79dd146fd83025cd7d69423e489d0370 100644
--- a/runtime/bin/dartutils.h
+++ b/runtime/bin/dartutils.h
@@ -130,19 +130,19 @@ class DartUtils {
                                            bool trace_loading,
                                            const char* package_root,
                                            const char* packages_file);
-  static void PrepareCoreLibrary(Dart_Handle core_lib,
+  static Dart_Handle PrepareCoreLibrary(Dart_Handle core_lib,
                                  Dart_Handle builtin_lib,
                                  bool is_service_isolate);
-  static void PrepareAsyncLibrary(Dart_Handle async_lib,
+  static Dart_Handle PrepareAsyncLibrary(Dart_Handle async_lib,
                                   Dart_Handle isolate_lib);
-  static void PrepareIOLibrary(Dart_Handle io_lib);
-  static void PrepareIsolateLibrary(Dart_Handle isolate_lib);
+  static Dart_Handle PrepareIOLibrary(Dart_Handle io_lib);
+  static Dart_Handle PrepareIsolateLibrary(Dart_Handle isolate_lib);
   static Dart_Handle PrepareForScriptLoading(const char* package_root,
                                              const char* packages_file,
                                              bool is_service_isolate,
                                              bool trace_loading,
                                              Dart_Handle builtin_lib);
-  static void SetupIOLibrary(const char* script_uri);
+  static Dart_Handle SetupIOLibrary(const char* script_uri);
 
   static bool PostNull(Dart_Port port_id);
   static bool PostInt32(Dart_Port port_id, int32_t value);