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

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
diff --git a/runtime/vm/assembler_x64.h b/runtime/vm/assembler_x64.h
index 66dc20c3e8f177aafb66eeb7d1161de48018a637..b26e5ea9947a50db3bfc9794ec3dec4b569aa878 100644
--- a/runtime/vm/assembler_x64.h
+++ b/runtime/vm/assembler_x64.h
@@ -519,6 +519,17 @@ class Assembler : public ValueObject {
   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) {