Use CHECK_ISOLATE instead of DARTSCOPE for Dart_GetNativeInstanceField after

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 33563)
+++ runtime/vm/object.cc	(working copy)
@@ -12123,7 +12123,10 @@
 
 
 bool Instance::IsValidFieldOffset(intptr_t offset) const {
-  const Class& cls = Class::Handle(clazz());
+  Isolate* isolate = Isolate::Current();
+  REUSABLE_CLASS_HANDLESCOPE(isolate);
+  Class& cls = isolate->ClassHandle();
+  cls = clazz();
   return (offset >= 0 && offset <= (cls.instance_size() - kWordSize));
 }