Make subtype test stubs isolate-specific.

runtime/vm/assembler_arm64.cc

Index: runtime/vm/assembler_arm64.cc
===================================================================
--- runtime/vm/assembler_arm64.cc	(revision 40703)
+++ runtime/vm/assembler_arm64.cc	(working copy)
@@ -1091,20 +1091,24 @@
 }
 
 
-void Assembler::LoadClassById(Register result, Register class_id, Register pp) {
+void Assembler::LoadClassById(Register result,
+                              Register class_id,
+                              Isolate* isolate,
+                              Register pp) {
   ASSERT(result != class_id);
-  LoadFieldFromOffset(result, CTX, Context::isolate_offset(), pp);
-  const intptr_t table_offset_in_isolate =
-      Isolate::class_table_offset() + ClassTable::table_offset();
-  LoadFromOffset(result, result, table_offset_in_isolate, pp);
+  LoadImmediate(result, isolate->ClassTableAddress(), pp);
+  LoadFromOffset(result, result, 0, pp);
   ldr(result, Address(result, class_id, UXTX, Address::Scaled));
 }
 
 
-void Assembler::LoadClass(Register result, Register object, Register pp) {
+void Assembler::LoadClass(Register result,
+                          Register object,
+                          Isolate* isolate,
+                          Register pp) {
   ASSERT(object != TMP);
   LoadClassId(TMP, object, pp);
-  LoadClassById(result, TMP, pp);
+  LoadClassById(result, TMP, isolate, pp);
 }