Make subtype test stubs isolate-specific.

runtime/vm/assembler_mips.cc

Index: runtime/vm/assembler_mips.cc
===================================================================
--- runtime/vm/assembler_mips.cc	(revision 40703)
+++ runtime/vm/assembler_mips.cc	(working copy)
@@ -669,31 +669,24 @@
 }
 
 
-void Assembler::LoadClassById(Register result, Register class_id) {
+void Assembler::LoadClassById(Register result,
+                              Register class_id,
+                              Isolate* isolate) {
   ASSERT(!in_delay_slot_);
   ASSERT(result != class_id);
-  lw(result, FieldAddress(CTX, Context::isolate_offset()));
-  const intptr_t table_offset_in_isolate =
-      Isolate::class_table_offset() + ClassTable::table_offset();
-  lw(result, Address(result, table_offset_in_isolate));
+  LoadImmediate(result, isolate->ClassTableAddress());
+  lw(result, Address(result, 0));
   sll(TMP, class_id, 2);
   addu(result, result, TMP);
   lw(result, Address(result));
 }
 
 
-void Assembler::LoadClass(Register result, Register object) {
+void Assembler::LoadClass(Register result, Register object, Isolate* isolate) {
   ASSERT(!in_delay_slot_);
   ASSERT(TMP != result);
   LoadClassId(TMP, object);
-
-  lw(result, FieldAddress(CTX, Context::isolate_offset()));
-  const intptr_t table_offset_in_isolate =
-      Isolate::class_table_offset() + ClassTable::table_offset();
-  lw(result, Address(result, table_offset_in_isolate));
-  sll(TMP, TMP, 2);
-  addu(result, result, TMP);
-  lw(result, Address(result));
+  LoadClassById(result, TMP, isolate);
 }