Fix GC problem with vm_handles in thread

runtime/vm/thread.cc

Index: runtime/vm/thread.cc
diff --git a/runtime/vm/thread.cc b/runtime/vm/thread.cc
index 9f08a73e107e21bae928e2c1fd2cd879346c8885..56c25af41b5d8144a628c5dd909f72cf0e734876 100644
--- a/runtime/vm/thread.cc
+++ b/runtime/vm/thread.cc
@@ -223,6 +223,11 @@ void Thread::ExitIsolate() {
   Thread* thread = Thread::Current();
   // TODO(koda): Audit callers; they should know whether they're in an isolate.
   if (thread == NULL || thread->isolate() == NULL) return;
+#if defined(DEBUG)
+  ASSERT(!thread->IsAnyReusableHandleScopeActive());
+#endif  // DEBUG
+  // Clear since GC will not visit the thread once it is unscheduled.
+  thread->ClearReusableHandles();
   Isolate* isolate = thread->isolate();
   Profiler::EndExecution(isolate);
   thread->Unschedule();
@@ -237,9 +242,6 @@ void Thread::ExitIsolate() {
   thread->isolate_ = NULL;
   ASSERT(Isolate::Current() == NULL);
   thread->heap_ = NULL;
-#if defined(DEBUG)
-  ASSERT(!thread->IsAnyReusableHandleScopeActive());
-#endif  // DEBUG
 }
 
 
@@ -348,6 +350,14 @@ C* Thread::AllocateReusableHandle() {
 }
 
 
+void Thread::ClearReusableHandles() {
+#define CLEAR_REUSABLE_HANDLE(object)                                          \
+  *object##_handle_ = object::null();
+  REUSABLE_HANDLE_LIST(CLEAR_REUSABLE_HANDLE)
+#undef CLEAR_REUSABLE_HANDLE
+}
+
+
 void Thread::VisitObjectPointers(ObjectPointerVisitor* visitor) {
   ASSERT(visitor != NULL);