Only lookup and delete the monitor wait data if it was allocated.

runtime/platform/thread_win.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 "platform/globals.h"
 #if defined(TARGET_OS_WINDOWS)
 
 #include "platform/thread.h"
 
 #include <process.h>  // NOLINT
@@ skipping 156 matching lines @@
   EnterCriticalSection(&data_.cs_);
 }
 
 
 void Monitor::Exit() {
   LeaveCriticalSection(&data_.cs_);
 }
 
 
 void MonitorWaitData::ThreadExit() {
-  uword raw_wait_data =
-    Thread::GetThreadLocal(MonitorWaitData::monitor_wait_data_key_);
-  if (raw_wait_data != 0) {
-    MonitorWaitData* wait_data =
-        reinterpret_cast<MonitorWaitData*>(raw_wait_data);
-    delete wait_data;
+  if (MonitorWaitData::monitor_wait_data_key_ !=
+      Thread::kUnsetThreadLocalKey) {
+    uword raw_wait_data =
+      Thread::GetThreadLocal(MonitorWaitData::monitor_wait_data_key_);
+    if (raw_wait_data != 0) {
+      MonitorWaitData* wait_data =
+          reinterpret_cast<MonitorWaitData*>(raw_wait_data);
+      delete wait_data;
+    }
   }
 }
 
 
 void MonitorData::AddWaiter(MonitorWaitData* wait_data) {
   // Add the MonitorWaitData object to the list of objects waiting for
   // this monitor.
   EnterCriticalSection(&waiters_cs_);
   if (waiters_tail_ == NULL) {
     ASSERT(waiters_head_ == NULL);
@@ skipping 152 matching lines @@
   // timeout before we signal it, that object will get an extra
   // signal. This will be treated as a spurious wake-up and is OK
   // since all uses of monitors should recheck the condition after a
   // Wait.
   data_.SignalAndRemoveAllWaiters();
 }
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)