Fix GC problem with vm_handles in thread

runtime/vm/thread_registry.h

 // Copyright (c) 2015, 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.
 
 #ifndef VM_THREAD_REGISTRY_H_
 #define VM_THREAD_REGISTRY_H_
 
 #include "vm/globals.h"
 #include "vm/growable_array.h"
 #include "vm/isolate.h"
@@ skipping 111 matching lines @@
   void VisitObjectPointers(ObjectPointerVisitor* visitor,
                            bool validate_frames) {
     MonitorLocker ml(monitor_);
     for (int i = 0; i < entries_.length(); ++i) {
       const Entry& entry = entries_[i];
       const Thread::State& state =
           entry.scheduled ? entry.thread->state_ : entry.state;
       if (state.zone != NULL) {
         state.zone->VisitObjectPointers(visitor);
       }
-      ASSERT(entry.thread != NULL);
-      entry.thread->VisitObjectPointers(visitor);
+      if (entry.scheduled) {
+        ASSERT(entry.thread != NULL);
+        entry.thread->VisitObjectPointers(visitor);
+      }
       // Iterate over all the stack frames and visit objects on the stack.
       StackFrameIterator frames_iterator(state.top_exit_frame_info,
                                          validate_frames);
       StackFrame* frame = frames_iterator.NextFrame();
       while (frame != NULL) {
         frame->VisitObjectPointers(visitor);
         frame = frames_iterator.NextFrame();
       }
     }
   }
@@ skipping 60 matching lines @@
   intptr_t remaining_;    // Number of threads yet to reach their safepoint.
   int64_t round_;         // Counter, to prevent missing updates to remaining_
                           // (see comments in CheckSafepointLocked).
 
   DISALLOW_COPY_AND_ASSIGN(ThreadRegistry);
 };
 
 }  // namespace dart
 
 #endif  // VM_THREAD_REGISTRY_H_