Make subtype test cache stub shared between isolates.

runtime/vm/assembler_mips.cc

Index: runtime/vm/assembler_mips.cc
diff --git a/runtime/vm/assembler_mips.cc b/runtime/vm/assembler_mips.cc
index b96cb1bd8c8d1a8b960e6f1f484dd3166719d8eb..da5a6dfef9413be67945bc219096ac4040ce0721 100644
--- a/runtime/vm/assembler_mips.cc
+++ b/runtime/vm/assembler_mips.cc
@@ -670,8 +670,10 @@ void Assembler::LoadClassId(Register result, Register object) {
 void Assembler::LoadClassById(Register result, Register class_id) {
   ASSERT(!in_delay_slot_);
   ASSERT(result != class_id);
-  LoadImmediate(result, Isolate::Current()->class_table()->TableAddress());
-  lw(result, Address(result, 0));
+  LoadIsolate(result);
+  const intptr_t offset =
+      Isolate::class_table_offset() + ClassTable::table_offset();
+  lw(result, Address(result, offset));
   sll(TMP, class_id, 2);
   addu(result, result, TMP);
   lw(result, Address(result));