Last snapshot bits to get working precompiled hello_world on x64.

runtime/vm/virtual_memory.cc

 // 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.
 
 #include "vm/virtual_memory.h"
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 
 namespace dart {
 
 bool VirtualMemory::InSamePage(uword address0, uword address1) {
   return (Utils::RoundDown(address0, PageSize()) ==
           Utils::RoundDown(address1, PageSize()));
 }
 
 
 void VirtualMemory::Truncate(intptr_t new_size, bool try_unmap) {
   ASSERT((new_size & (PageSize() - 1)) == 0);
   ASSERT(new_size <= size());
   if (try_unmap &&
       (reserved_size_ == size()) &&  /* Don't create holes in reservation. */
       FreeSubSegment(reinterpret_cast<void*>(start() + new_size),
                      size() - new_size)) {
     reserved_size_ = new_size;
   }
   region_.Subregion(region_, 0, new_size);
 }
 
+
+VirtualMemory* VirtualMemory::ForInstructionsSnapshot(void* pointer,
+                                                      uword size) {
+  // Memory for precompilated instructions was allocated by the embedder, so
+  // create a VirtualMemory without allocating.
+  MemoryRegion region(pointer, size);
+  return new VirtualMemory(region);
+}
+
 }  // namespace dart