VM: Fix for deadlock with background tasks.

runtime/vm/gc_sweeper.cc

 // Copyright (c) 2011, 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/gc_sweeper.h"
 
 #include "vm/freelist.h"
 #include "vm/globals.h"
 #include "vm/heap.h"
 #include "vm/lockers.h"
@@ skipping 93 matching lines @@
         first_(first),
         last_(last),
         freelist_(freelist) {
     ASSERT(task_isolate_ != NULL);
     ASSERT(first_ != NULL);
     ASSERT(old_space_ != NULL);
     ASSERT(last_ != NULL);
     ASSERT(freelist_ != NULL);
     MonitorLocker ml(old_space_->tasks_lock());
     old_space_->set_tasks(old_space_->tasks() + 1);
-    ml.Notify();
   }
 
   virtual void Run() {
     bool result = Thread::EnterIsolateAsHelper(task_isolate_,
                                                Thread::kSweeperTask);
     ASSERT(result);
     {
       Thread* thread = Thread::Current();
       TIMELINE_FUNCTION_GC_DURATION(thread, "SweeperTask");
       GCSweeper sweeper;
@@ skipping 47 matching lines @@
   SweeperTask* task =
       new SweeperTask(isolate,
                       isolate->heap()->old_space(),
                       first, last,
                       freelist);
   ThreadPool* pool = Dart::thread_pool();
   pool->Run(task);
 }
 
 }  // namespace dart