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

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index 34a8d33f97af77b721ac7514d7c0b81924c5279b..66894b59bb516d37e98910bf3142f944943142b6 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -1344,11 +1344,10 @@ 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