Add VerifiedMemory helper for write-barrier verification.

runtime/vm/virtual_memory.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 VM_VIRTUAL_MEMORY_H_
 #define VM_VIRTUAL_MEMORY_H_
 
 #include "platform/utils.h"
 #include "vm/globals.h"
 #include "vm/memory_region.h"
@@ skipping 42 matching lines @@
   }
 
   static intptr_t PageSize() {
     ASSERT(page_size_ != 0);
     ASSERT(Utils::IsPowerOfTwo(page_size_));
     return page_size_;
   }
 
   static bool InSamePage(uword address0, uword address1);
 
-  // Truncate this virtual memory segment.
+  // Truncate this virtual memory segment. If try_unmap is false, the
+  // memory beyond the new end is still accessible, but will be returned
+  // upon destruction.
   void Truncate(intptr_t new_size, bool try_unmap = true);
 
+  // Commit a reserved memory area, so that the memory can be accessed.
+  bool Commit(uword addr, intptr_t size, bool is_executable);
+
  private:
   static VirtualMemory* ReserveInternal(intptr_t size);
 
   // Free a sub segment. On operating systems that support it this
   // can give back the virtual memory to the system. Returns true on success.
   static bool FreeSubSegment(void* address, intptr_t size);
 
   // This constructor is only used internally when reserving new virtual spaces.
   // It does not reserve any virtual address space on its own.
   explicit VirtualMemory(const MemoryRegion& region) :
       region_(region.pointer(), region.size()),
       reserved_size_(region.size()) { }
 
-  // Commit a reserved memory area, so that the memory can be accessed.
-  bool Commit(uword addr, intptr_t size, bool is_executable);
-
   MemoryRegion region_;
 
   // The size of the underlying reservation not yet given back to the OS.
   // Its start coincides with region_, but its size might not, due to Truncate.
   intptr_t reserved_size_;
 
   static uword page_size_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(VirtualMemory);
 };
 
 }  // namespace dart
 
 #endif  // VM_VIRTUAL_MEMORY_H_