Fixes shutdown crashes.

runtime/vm/virtual_memory_macos.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/globals.h"
 #if defined(TARGET_OS_MACOS)
 
 #include "vm/virtual_memory.h"
 
 #include <sys/mman.h>  // NOLINT
@@ skipping 35 matching lines @@
     return;
   }
 
   if (munmap(address, size) != 0) {
     FATAL("munmap failed\n");
   }
 }
 
 
 VirtualMemory::~VirtualMemory() {
-  unmap(address(), reserved_size_);
+  if (!embedder_allocated()) {
+    unmap(address(), reserved_size_);
+  }
 }
 
 
 bool VirtualMemory::FreeSubSegment(void* address, intptr_t size) {
   unmap(address, size);
   return true;
 }
 
 
 bool VirtualMemory::Commit(uword addr, intptr_t size, bool executable) {
@@ skipping 33 matching lines @@
       break;
   }
   return (mprotect(reinterpret_cast<void*>(page_address),
                    end_address - page_address,
                    prot) == 0);
 }
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)