Fix race condition with HasMutatorThread that was being used without a lock.

runtime/vm/virtual_memory_linux.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_LINUX)
 
 #include "vm/virtual_memory.h"
 
 #include <sys/mman.h>  // NOLINT
@@ skipping 64 matching lines @@
                        -1, 0);
   if (address == MAP_FAILED) {
     return false;
   }
   return true;
 }
 
 
 bool VirtualMemory::Protect(void* address, intptr_t size, Protection mode) {
   ASSERT(Thread::Current()->IsMutatorThread() ||
-         !Isolate::Current()->HasMutatorThread() ||
          Isolate::Current()->mutator_thread()->IsAtSafepoint());
   uword start_address = reinterpret_cast<uword>(address);
   uword end_address = start_address + size;
   uword page_address = Utils::RoundDown(start_address, PageSize());
   int prot = 0;
   switch (mode) {
     case kNoAccess:
       prot = PROT_NONE;
       break;
     case kReadOnly:
@@ skipping 11 matching lines @@
   }
   return (mprotect(reinterpret_cast<void*>(page_address),
                    end_address - page_address,
                    prot) == 0);
 }
 
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_LINUX)