WIP: Implement ES6 Spread-calls

src/x64/codegen-x64.h

Index: src/x64/codegen-x64.h
diff --git a/src/x64/codegen-x64.h b/src/x64/codegen-x64.h
index 0a551eef5c3de55e0fa39ec5a52c40670378a4d4..96470dbb5b0bb7bbf32f007d4d52cec3d95ce267 100644
--- a/src/x64/codegen-x64.h
+++ b/src/x64/codegen-x64.h
@@ -99,6 +99,7 @@ class StackArgumentsAccessor BASE_EMBEDDED {
     DCHECK(receiver_mode_ == ARGUMENTS_CONTAIN_RECEIVER);
     return GetArgumentOperand(0);
   }
+  const Register& argument_count_reg() const { return argument_count_reg_; }
 
  private:
   const Register base_reg_;
@@ -106,8 +107,6 @@ class StackArgumentsAccessor BASE_EMBEDDED {
   const int argument_count_immediate_;
   const StackArgumentsAccessorReceiverMode receiver_mode_;
   const int extra_displacement_to_last_argument_;
-
-  DISALLOW_IMPLICIT_CONSTRUCTORS(StackArgumentsAccessor);
 };