Both the mutator and the background compiler may be waiting on the sweeper task.

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 134 matching lines @@
         if (page == last_) break;
         page = next_page;
       }
     }
     // Exit isolate cleanly *before* notifying it, to avoid shutdown race.
     Thread::ExitIsolateAsHelper();
     // This sweeper task is done. Notify the original isolate.
     {
       MonitorLocker ml(old_space_->tasks_lock());
       old_space_->set_tasks(old_space_->tasks() - 1);
-      ml.Notify();
+      ml.NotifyAll();
     }
   }
 
  private:
   Isolate* task_isolate_;
   PageSpace* old_space_;
   HeapPage* first_;
   HeapPage* last_;
   FreeList* freelist_;
 };
 
 
 void GCSweeper::SweepConcurrent(Isolate* isolate,
                                 HeapPage* first,
                                 HeapPage* last,
                                 FreeList* freelist) {
   SweeperTask* task =
       new SweeperTask(isolate,
                       isolate->heap()->old_space(),
                       first, last,
                       freelist);
   ThreadPool* pool = Dart::thread_pool();
   pool->Run(task);
 }
 
 }  // namespace dart