Implement safepointing of threads

runtime/vm/gc_sweeper.cc

Index: runtime/vm/gc_sweeper.cc
diff --git a/runtime/vm/gc_sweeper.cc b/runtime/vm/gc_sweeper.cc
index a192138b4c6f5a72ddf92f2360cb4ea8c7ded1e4..0dcf62fb276c7b944f512602fd4847b59aaf6d27 100644
--- a/runtime/vm/gc_sweeper.cc
+++ b/runtime/vm/gc_sweeper.cc
@@ -9,8 +9,8 @@
 #include "vm/heap.h"
 #include "vm/lockers.h"
 #include "vm/pages.h"
+#include "vm/safepoint.h"
 #include "vm/thread_pool.h"
-#include "vm/thread_registry.h"
 
 namespace dart {
 
@@ -116,13 +116,14 @@ class SweeperTask : public ThreadPool::Task {
   virtual void Run() {
     bool result = Thread::EnterIsolateAsHelper(task_isolate_);
     ASSERT(result);
+    Thread* thread = Thread::Current();
     GCSweeper sweeper;
 
     HeapPage* page = first_;
     HeapPage* prev_page = NULL;
 
     while (page != NULL) {
-      task_isolate_->thread_registry()->CheckSafepoint();
+      thread->CheckForSafepoint();
       HeapPage* next_page = page->next();
       ASSERT(page->type() == HeapPage::kData);
       bool page_in_use = sweeper.SweepPage(page, freelist_, false);