Fixes shutdown crashes.

runtime/vm/virtual_memory.h

Index: runtime/vm/virtual_memory.h
diff --git a/runtime/vm/virtual_memory.h b/runtime/vm/virtual_memory.h
index 44f34910e891741487816ce99bf05c66b6c6c45b..cf133f2e636a81094bc9d6de82ea1e933a188bac 100644
--- a/runtime/vm/virtual_memory.h
+++ b/runtime/vm/virtual_memory.h
@@ -69,6 +69,8 @@ class VirtualMemory {
   // Commit a reserved memory area, so that the memory can be accessed.
   bool Commit(uword addr, intptr_t size, bool is_executable);
 
+  bool dummy() const { return dummy_; }
+
   static VirtualMemory* ForInstructionsSnapshot(void* pointer, uword size);
 
  private:
@@ -92,6 +94,9 @@ class VirtualMemory {
 
   static uword page_size_;
 
+  // Dummy region for instructions snapshot.
+  bool dummy_;
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(VirtualMemory);
 };