Last snapshot bits to get working precompiled hello_world on x64.

runtime/vm/virtual_memory.cc

Index: runtime/vm/virtual_memory.cc
diff --git a/runtime/vm/virtual_memory.cc b/runtime/vm/virtual_memory.cc
index 050c83d7312577aac313348da72c60079cdcdde5..2f2c1bc855681688a6e8a8e7d3d6af33af7b7c64 100644
--- a/runtime/vm/virtual_memory.cc
+++ b/runtime/vm/virtual_memory.cc
@@ -27,4 +27,13 @@ void VirtualMemory::Truncate(intptr_t new_size, bool try_unmap) {
   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