Restructure/fix thread pointer visiting; added some asserts.

runtime/vm/thread.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_H_
 #define VM_THREAD_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/atomic.h"
@@ skipping 503 matching lines @@
 
   void CheckForSafepoint() {
     if (IsSafepointRequested()) {
       BlockForSafepoint();
     }
   }
 
   Thread* next() const { return next_; }
 
   // Visit all object pointers.
-  void VisitObjectPointers(ObjectPointerVisitor* visitor);
+  void VisitObjectPointers(ObjectPointerVisitor* visitor, bool validate_frames);
 
   bool IsValidLocalHandle(Dart_Handle object) const;
   int CountLocalHandles() const;
   int ZoneSizeInBytes() const;
   void UnwindScopes(uword stack_marker);
 
   void InitVMConstants();
 
  private:
   template<class T> T* AllocateReusableHandle();
@@ skipping 111 matching lines @@
 // Disable thread interrupts.
 class DisableThreadInterruptsScope : public StackResource {
  public:
   explicit DisableThreadInterruptsScope(Thread* thread);
   ~DisableThreadInterruptsScope();
 };
 
 }  // namespace dart
 
 #endif  // VM_THREAD_H_