Eliminate fake-Isolate workaround from concurrent sweeper.

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
===================================================================
--- runtime/vm/gc_marker.cc	(revision 44565)
+++ runtime/vm/gc_marker.cc	(working copy)
@@ -230,7 +230,8 @@
       return;
     }
     if (RawObject::IsVariableSizeClassId(raw_obj->GetClassId())) {
-      class_table_->UpdateLiveOld(raw_obj->GetClassId(), raw_obj->Size());
+      class_table_->UpdateLiveOld(raw_obj->GetClassId(),
+                                  raw_obj->Size(class_table_));
     } else {
       class_table_->UpdateLiveOld(raw_obj->GetClassId(), 0);
     }
@@ -425,7 +426,7 @@
       marked_bytes_ += raw_obj->VisitPointers(visitor);
     } else {
       RawWeakProperty* raw_weak = reinterpret_cast<RawWeakProperty*>(raw_obj);
-      marked_bytes_ += raw_weak->Size();
+      marked_bytes_ += raw_weak->Size(isolate->class_table());
       ProcessWeakProperty(raw_weak, visitor);
     }
   }