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

runtime/vm/assembler_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_ASSEMBLER_X64_H_
 #define VM_ASSEMBLER_X64_H_
 
 #ifndef VM_ASSEMBLER_H_
 #error Do not include assembler_x64.h directly; use assembler.h instead.
 #endif
@@ skipping 501 matching lines @@
     lock();
     cmpxchgl(address, reg);
   }
 
   void EnterFrame(intptr_t frame_space);
   void LeaveFrame();
 
   void CallRuntime(const RuntimeEntry& entry);
 
   /*
+   * Loading and comparing classes of objects.
+   */
+  void LoadClassId(Register result, Register object);
+
+  void LoadClassById(Register result, Register class_id);
+
+  void LoadClass(Register result, Register object);
+
+  void CompareClassId(Register object, intptr_t class_id);
+
+  /*
    * Misc. functionality.
    */
   void SmiTag(Register reg) {
     addq(reg, reg);
   }
 
   void SmiUntag(Register reg) {
     sarq(reg, Immediate(kSmiTagSize));
   }
 
@@ skipping 104 matching lines @@
 }
 
 
 inline void Assembler::EmitOperandSizeOverride() {
   EmitUint8(0x66);
 }
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_X64_H_