Eliminate fake-Isolate workaround from concurrent sweeper.

runtime/vm/gc_sweeper.h

 // 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.
 
 #ifndef VM_GC_SWEEPER_H_
 #define VM_GC_SWEEPER_H_
 
 #include "vm/globals.h"
 
 namespace dart {
 
 // Forward declarations.
+class ClassTable;
 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.
 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
   // all the unmarked objects to the freelist. Whether the freelist is
   // pre-locked is indicated by the locked parameter.
   // Returns true if the page is in use. Freelist is untouched if page is not
   // in use.
   bool SweepPage(HeapPage* page, FreeList* freelist, bool locked);
 
   // Returns the number of words from page->object_start() to the end of the
   // 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:
+  const ClassTable* class_table_;
 };
 
 }  // namespace dart
 
 #endif  // VM_GC_SWEEPER_H_