Make subtype test cache stub shared between isolates.

runtime/vm/class_table.h

Index: runtime/vm/class_table.h
diff --git a/runtime/vm/class_table.h b/runtime/vm/class_table.h
index 384b3d11ae9759922bda686be1f26878b7a6998a..9a23630083d8b04565ade17175316fd9c1a1d914 100644
--- a/runtime/vm/class_table.h
+++ b/runtime/vm/class_table.h
@@ -178,6 +178,7 @@ class ClassTable {
 
   void PrintToJSONObject(JSONObject* object);
 
+  // Used by the generated code.
   static intptr_t table_offset() {
     return OFFSET_OF(ClassTable, table_);
   }
@@ -194,11 +195,6 @@ class ClassTable {
   void UpdatePromoted();
 
   // Used by the generated code.
-  uword TableAddress() {
-    return reinterpret_cast<uword>(&table_);
-  }
-
-  // Used by the generated code.
   uword PredefinedClassHeapStatsTableAddress() {
     return reinterpret_cast<uword>(predefined_class_heap_stats_table_);
   }