- Add and enable concurrent sweeper.

runtime/vm/gc_sweeper.h

Index: runtime/vm/gc_sweeper.h
===================================================================
--- runtime/vm/gc_sweeper.h	(revision 39573)
+++ runtime/vm/gc_sweeper.h	(working copy)
@@ -13,6 +13,8 @@
 class FreeList;
 class Heap;
 class HeapPage;
+class Isolate;
+class PageSpace;
 
 // The class GCSweeper is used to visit the heap after marking to reclaim unused
 // memory.
@@ -30,6 +32,12 @@
   // last marked object.
   intptr_t SweepLargePage(HeapPage* page);
 
+  // Sweep the regular sized data pages between first and last inclusive.
+  static void SweepConcurrent(Isolate* isolate,
+                              HeapPage* first,
+                              HeapPage* last,
+                              FreeList* freelist);
+
  private:
   Heap* heap_;