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

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
diff --git a/runtime/vm/intermediate_language_x64.cc b/runtime/vm/intermediate_language_x64.cc
index aeb58d946e04806232e821475836cbdd2ffea35f..7530134dfb0c7e4e3f8f442a7fb234fcda129b27 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -712,8 +712,9 @@ void InstantiateTypeArgumentsComp::EmitNativeCode(FlowGraphCompiler* compiler) {
     // No need to check the instantiator ('instantiator_reg') for null here,
     // because a null instantiator will have the wrong class (Null instead of
     // TypeArguments).
-    __ LoadObject(temp_reg, Class::ZoneHandle(Object::type_arguments_class()));
-    __ cmpq(temp_reg, FieldAddress(instantiator_reg, Object::class_offset()));
+    __ CompareClassOfObject(instantiator_reg,
+                            Class::Handle(Object::type_arguments_class()),
+                            temp_reg);
     __ j(NOT_EQUAL, &type_arguments_uninstantiated, Assembler::kNearJump);
     Immediate arguments_length =
         Immediate(Smi::RawValue(type_arguments().Length()));
@@ -801,8 +802,9 @@ void ExtractConstructorInstantiatorComp::EmitNativeCode(
     // matching length and, if so, use it as the instantiated type_arguments.
     // No need to check the instantiator (RAX) for null here, because a null
     // instantiator will have the wrong class (Null instead of TypeArguments).
-    __ LoadObject(temp_reg, Class::ZoneHandle(Object::type_arguments_class()));
-    __ cmpq(temp_reg, FieldAddress(instantiator_reg, Object::class_offset()));
+    __ CompareClassOfObject(instantiator_reg,
+                            Class::Handle(Object::type_arguments_class()),
+                            temp_reg);
     __ j(NOT_EQUAL, &done, Assembler::kNearJump);
     Immediate arguments_length =
         Immediate(Smi::RawValue(type_arguments().Length()));