Implement safepointing of threads

runtime/vm/bootstrap.cc

Index: runtime/vm/bootstrap.cc
diff --git a/runtime/vm/bootstrap.cc b/runtime/vm/bootstrap.cc
index f7bd22db7107ed9a0783d0dac77e1be5a7a8b221..1868f07a3034bf1fd5d2ff207ba7b0a86e86e56a 100644
--- a/runtime/vm/bootstrap.cc
+++ b/runtime/vm/bootstrap.cc
@@ -209,6 +209,9 @@ static Dart_Handle BootstrapLibraryTagHandler(Dart_LibraryTag tag,
                                               Dart_Handle uri) {
   Thread* thread = Thread::Current();
   Zone* zone = thread->zone();
+  // This handler calls into the VM directly and does not use the Dart
+  // API so we transition back to VM.
+  TransitionNativeToVM transition(thread);
   if (!Dart_IsLibrary(library)) {
     return Api::NewError("not a library");
   }