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

runtime/vm/dart_api_impl.cc

 // Copyright (c) 2013, 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 "include/dart_api.h"
 #include "include/dart_mirrors_api.h"
 #include "include/dart_native_api.h"
 
 #include "platform/assert.h"
 #include "lib/stacktrace.h"
@@ skipping 1326 matching lines @@
   DARTSCOPE(Thread::Current());
   Isolate* I = T->isolate();
   return Api::NewHandle(T, String::New(I->name()));
 }
 
 
 DART_EXPORT void Dart_EnterIsolate(Dart_Isolate isolate) {
   CHECK_NO_ISOLATE(Isolate::Current());
   // TODO(16615): Validate isolate parameter.
   Isolate* iso = reinterpret_cast<Isolate*>(isolate);
-  if (iso->HasMutatorThread()) {
-    FATAL("Multiple mutators within one isolate is not supported.");
-  }
   if (!Thread::EnterIsolate(iso)) {
-    FATAL("Unable to Enter Isolate as Dart VM is shutting down");
+    FATAL("Unable to Enter Isolate : "
+          "Multiple mutators entering an isolate / "
+          "Dart VM is shutting down");
   }
   // A Thread structure has been associated to the thread, we do the
   // safepoint transition explicity here instead of using the
   // TransitionXXX scope objects as the reverse transition happens
   // outside this scope in Dart_ExitIsolate/Dart_ShutdownIsolate.
   Thread* T = Thread::Current();
   T->set_execution_state(Thread::kThreadInNative);
   T->EnterSafepoint();
 }
 
@@ skipping 5134 matching lines @@
 
 DART_EXPORT bool Dart_IsPrecompiledRuntime() {
 #if defined(DART_PRECOMPILED_RUNTIME)
   return true;
 #else
   return false;
 #endif
 }
 
 }  // namespace dart