1. Avoid potential dead lock due to lock-order-inversion

runtime/vm/heap.h

 // Copyright (c) 2012, 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 RUNTIME_VM_HEAP_H_
 #define RUNTIME_VM_HEAP_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/flags.h"
@@ skipping 96 matching lines @@
   RawObject* FindNewObject(FindObjectVisitor* visitor) const;
   RawObject* FindObject(FindObjectVisitor* visitor) const;
 
   void CollectGarbage(Space space);
   void CollectGarbage(Space space, ApiCallbacks api_callbacks, GCReason reason);
   void CollectAllGarbage();
   bool NeedsGarbageCollection() const {
     return old_space_.NeedsGarbageCollection();
   }
 
-#if defined(DEBUG)
-  void WaitForSweeperTasks();
-#endif
+  void WaitForSweeperTasks(Thread* thread);
 
   // Enables growth control on the page space heaps.  This should be
   // called before any user code is executed.
   void InitGrowthControl();
   void EnableGrowthControl() { SetGrowthControlState(true); }
   void DisableGrowthControl() { SetGrowthControlState(false); }
   void SetGrowthControlState(bool state);
   bool GrowthControlState();
 
   // Protect access to the heap. Note: Code pages are made
@@ skipping 258 matching lines @@
 // Note: During this scope, the code pages are non-executable.
 class WritableVMIsolateScope : StackResource {
  public:
   explicit WritableVMIsolateScope(Thread* thread);
   ~WritableVMIsolateScope();
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_HEAP_H_