Thread fixes for shutdown.

runtime/vm/os_thread.cc

 // Copyright (c) 2015, 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/os_thread.h"
 
 #include "vm/atomic.h"
 #include "vm/lockers.h"
 #include "vm/log.h"
 #include "vm/thread_interrupter.h"
@@ skipping 18 matching lines @@
     timeline_block_(NULL),
     thread_list_next_(NULL),
     thread_interrupt_disabled_(1),  // Thread interrupts disabled by default.
     log_(new class Log()),
     stack_base_(0),
     thread_(NULL) {
 }
 
 
 OSThread* OSThread::CreateOSThread() {
-  ASSERT(thread_list_lock_ != NULL);
-  MutexLocker ml(thread_list_lock_);
-  if (!creation_enabled_) {
+  if (thread_list_lock_ == NULL) {
     return NULL;
   }
-  OSThread* os_thread = new OSThread();
-  AddThreadToListLocked(os_thread);
-  return os_thread;
+  {
+    MutexLocker ml(thread_list_lock_);
+    if (!creation_enabled_) {
+      return NULL;
+    }
+    OSThread* os_thread = new OSThread();
+    AddThreadToListLocked(os_thread);
+    return os_thread;
+  }
 }
 
 
 OSThread::~OSThread() {
   RemoveThreadFromList(this);
   delete log_;
   log_ = NULL;
   if (Timeline::recorder() != NULL) {
     Timeline::recorder()->FinishBlock(timeline_block_);
   }
@@ skipping 207 matching lines @@
 
 OSThread* OSThreadIterator::Next() {
   ASSERT(OSThread::thread_list_lock_ != NULL);
   ASSERT(OSThread::thread_list_lock_->IsOwnedByCurrentThread());
   OSThread* current = next_;
   next_ = next_->thread_list_next_;
   return current;
 }
 
 }  // namespace dart