In generated code for x64 don't load object's class directly from class_ field.

runtime/vm/flow_graph_compiler_x64.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_FLOW_GRAPH_COMPILER_X64_H_
 #define VM_FLOW_GRAPH_COMPILER_X64_H_
 
 #ifndef VM_FLOW_GRAPH_COMPILER_H_
 #error Include flow_graph_compiler.h instead of flow_graph_compiler_x64.h.
 #endif
@@ skipping 115 matching lines @@
   void GenerateCallRuntime(intptr_t cid,
                            intptr_t token_index,
                            intptr_t try_index,
                            const RuntimeEntry& entry);
   void AddCurrentDescriptor(PcDescriptors::Kind kind,
                             intptr_t cid,
                             intptr_t token_index,
                             intptr_t try_index);
 
   // Type checking helper methods.
-  void CheckClasses(const GrowableArray<const Class*>& classes,
+  void CheckClassIds(const GrowableArray<intptr_t>& class_ids,
                     Label* is_instance_lbl,
                     Label* is_not_instance_lbl);
   RawSubtypeTestCache* GenerateInlineInstanceof(intptr_t cid,
                                                 intptr_t token_index,
                                                 const AbstractType& type,
                                                 Label* is_instance,
                                                 Label* is_not_instance);
 
   RawSubtypeTestCache* GenerateInstantiatedTypeWithArgumentsTest(
       intptr_t cid,
@@ skipping 57 matching lines @@
   ExceptionHandlerList* exception_handlers_list_;
   GrowableArray<DeoptimizationStub*> deopt_stubs_;
   const bool is_optimizing_;
 
   DISALLOW_COPY_AND_ASSIGN(FlowGraphCompiler);
 };
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_COMPILER_X64_H_