Eliminate fake-Isolate workaround from concurrent sweeper.

runtime/vm/gc_sweeper.h

Index: runtime/vm/gc_sweeper.h
===================================================================
--- runtime/vm/gc_sweeper.h	(revision 44565)
+++ runtime/vm/gc_sweeper.h	(working copy)
@@ -10,6 +10,7 @@
 namespace dart {
 
 // Forward declarations.
+class ClassTable;
 class FreeList;
 class Heap;
 class HeapPage;
@@ -20,7 +21,8 @@
 // memory.
 class GCSweeper {
  public:
-  GCSweeper() {}
+  explicit GCSweeper(const ClassTable* class_table)
+      : class_table_(class_table) {}
   ~GCSweeper() {}
 
   // Sweep the memory area for the page while clearing the mark bits and adding
@@ -39,6 +41,9 @@
                               HeapPage* first,
                               HeapPage* last,
                               FreeList* freelist);
+
+ private:
+  const ClassTable* class_table_;
 };
 
 }  // namespace dart