Implement safepointing of threads

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 14e54ef0f8531a7356c80c7b8b86034117d7fdf6..14cdfcef3ce3bc7bf11a3eaab83bc7cc3a7ebe27 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -2773,8 +2773,7 @@ class CHACodeArray : public WeakCodeReferences {
 #if defined(DEBUG)
 static bool IsMutatorOrAtSafepoint() {
   Thread* thread = Thread::Current();
-  return thread->IsMutatorThread() ||
-         thread->isolate()->thread_registry()->AtSafepoint();
+  return thread->IsMutatorThread() || thread->IsAtSafepoint();
 }
 #endif
 
@@ -10684,7 +10683,6 @@ bool LibraryPrefix::LoadLibrary() const {
   } else if (deferred_lib.LoadNotStarted()) {
     Thread* thread = Thread::Current();
     Isolate* isolate = thread->isolate();
-    Api::Scope api_scope(thread);
     Zone* zone = thread->zone();
     deferred_lib.SetLoadRequested();
     const GrowableObjectArray& pending_deferred_loads =
@@ -10693,9 +10691,13 @@ bool LibraryPrefix::LoadLibrary() const {
     pending_deferred_loads.Add(deferred_lib);
     const String& lib_url = String::Handle(zone, deferred_lib.url());
     Dart_LibraryTagHandler handler = isolate->library_tag_handler();
-    handler(Dart_kImportTag,
-            Api::NewHandle(thread, importer()),
-            Api::NewHandle(thread, lib_url.raw()));
+    {
+      TransitionVMToNative transition(thread);
+      Api::Scope api_scope(thread);
+      handler(Dart_kImportTag,
+              Api::NewHandle(thread, importer()),
+              Api::NewHandle(thread, lib_url.raw()));
+    }
   } else {
     // Another load request is in flight.
     ASSERT(deferred_lib.LoadRequested());