Make subtype test stubs isolate-specific.

runtime/vm/flow_graph_compiler_arm64.cc

Index: runtime/vm/flow_graph_compiler_arm64.cc
===================================================================
--- runtime/vm/flow_graph_compiler_arm64.cc	(revision 40703)
+++ runtime/vm/flow_graph_compiler_arm64.cc	(working copy)
@@ -368,7 +368,7 @@
   }
   if (type.IsFunctionType()) {
     // Check if instance is a closure.
-    __ LoadClassById(R3, kClassIdReg, PP);
+    __ LoadClassById(R3, kClassIdReg, isolate(), PP);
     __ LoadFieldFromOffset(R3, R3, Class::signature_function_offset(), PP);
     __ CompareObject(R3, Object::null_object(), PP);
     __ b(is_instance_lbl, NE);
@@ -403,7 +403,7 @@
     Label* is_not_instance_lbl) {
   __ Comment("Subtype1TestCacheLookup");
   const Register kInstanceReg = R0;
-  __ LoadClass(R1, kInstanceReg, PP);
+  __ LoadClass(R1, kInstanceReg, isolate(), PP);
   // R1: instance class.
   // Check immediate superclass equality.
   __ LoadFieldFromOffset(R2, R1, Class::super_type_offset(), PP);