Remove fake isolate from concurrent sweeper and extend lifetime of VM threads.

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 95 matching lines @@
     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() {
-    Thread::EnterIsolate(task_isolate_);
+    Thread::EnterIsolateAsHelper(task_isolate_);
     GCSweeper sweeper;
 
     HeapPage* page = first_;
     HeapPage* prev_page = NULL;
 
     while (page != NULL) {
       HeapPage* next_page = page->next();
       ASSERT(page->type() == HeapPage::kData);
       bool page_in_use = sweeper.SweepPage(page, freelist_, false);
       if (page_in_use) {
         prev_page = page;
       } else {
         old_space_->FreePage(page, prev_page);
       }
       {
         // Notify the mutator thread that we have added elements to the free
         // list or that more capacity is available.
         MonitorLocker ml(old_space_->tasks_lock());
         ml.Notify();
       }
       if (page == last_) break;
       page = next_page;
     }
     // 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();
     }
-    Thread::ExitIsolate();
-    delete task_isolate_;
+    Thread::ExitIsolateAsHelper();
   }
 
  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->ShallowCopy(),
+      new SweeperTask(isolate,
                       isolate->heap()->old_space(),
                       first, last,
                       freelist);
   ThreadPool* pool = Dart::thread_pool();
   pool->Run(task);
 }
 
 }  // namespace dart