Eliminate fake-Isolate workaround from concurrent sweeper.

runtime/vm/raw_object.cc

Index: runtime/vm/raw_object.cc
===================================================================
--- runtime/vm/raw_object.cc	(revision 44565)
+++ runtime/vm/raw_object.cc	(working copy)
@@ -65,14 +65,15 @@
 
 
 intptr_t RawObject::SizeFromClass() const {
-  Isolate* isolate = Isolate::Current();
-  NoHandleScope no_handles(isolate);
+  return SizeFromClass(Isolate::Current()->class_table());
+}
 
+
+intptr_t RawObject::SizeFromClass(const ClassTable* class_table) const {
   // Only reasonable to be called on heap objects.
   ASSERT(IsHeapObject());
 
   intptr_t class_id = GetClassId();
-  ClassTable* class_table = isolate->class_table();
 #if defined(DEBUG)
   if (!class_table->IsValidIndex(class_id) ||
       !class_table->HasValidClassAt(class_id)) {