Added code to trace zone and handles creation/deletion under flags

vm/handles.h

 // Copyright (c) 2011, 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_HANDLES_H_
 #define VM_HANDLES_H_
 
 #include "vm/allocation.h"
 #include "vm/flags.h"
+#include "vm/os.h"
 
 namespace dart {
 
 // Handles are used in the Dart Virtual Machine to ensure that access
 // to dart objects in the virtual machine code is done in a
 // Garbage Collection safe manner.
 //
 // The class Handles is the basic type that implements creation of handles and
 // manages their life cycle (allocated either in the current zone or
 // current handle scope).
@@ skipping 32 matching lines @@
 //   NOHANDLESCOPE(isolate);
 //   ....
 //   ....
 // }
 
 
 // Forward declarations.
 class ObjectPointerVisitor;
 
 DECLARE_FLAG(bool, verify_handles);
+DECLARE_DEBUG_FLAG(bool, trace_handles);
 
 class HandleVisitor {
  public:
   virtual void VisitHandle(uword addr) = 0;
 
   virtual ~HandleVisitor() {
   }
 };
 
 
@@ skipping 175 matching lines @@
 static const int kVMHandlesPerChunk = 64;
 static const int kOffsetOfRawPtr = kWordSize;
 class VMHandles : public Handles<kVMHandleSizeInWords,
                                  kVMHandlesPerChunk,
                                  kOffsetOfRawPtr> {
  public:
   static const int kOffsetOfRawPtrInHandle = kOffsetOfRawPtr;
 
   VMHandles() : Handles<kVMHandleSizeInWords,
                         kVMHandlesPerChunk,
-                        kOffsetOfRawPtr>() { }
+                        kOffsetOfRawPtr>() {
+#ifdef DEBUG
+    if (FLAG_trace_handles) {
+      OS::PrintErr("*** Starting a new VM handle block 0x%"Px"\n",
+                   reinterpret_cast<intptr_t>(this));
+    }
+#endif
+  }
   ~VMHandles();
 
   // Visit all object pointers stored in the various handles.
   void VisitObjectPointers(ObjectPointerVisitor* visitor);
 
   // Allocates a handle in the current handle scope. This handle is valid only
   // in the current handle scope and is destroyed when the current handle
   // scope ends.
   static uword AllocateHandle(Isolate* isolate);
 
@@ skipping 72 matching lines @@
 };
 #endif  // defined(DEBUG)
 
 // Macro to start a no handles scope in the code.
 #define NOHANDLESCOPE(isolate)                                                \
     dart::NoHandleScope no_vm_internal_handles_scope_(isolate);
 
 }  // namespace dart
 
 #endif  // VM_HANDLES_H_