Cleanups: addressed your comments from https://codereview.chromium.org/14962008/

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 22880)
+++ runtime/vm/object.h	(working copy)
@@ -4096,12 +4096,12 @@
     return RoundedAllocationSize(sizeof(RawBigint) + (len * kBytesPerElement));
   }
 
-  RawBigint* ArithmeticOp(Token::Kind operation, const Bigint& other) const;
-
  protected:
   // Only Integer::NewXXX is allowed to call Bigint::NewXXX directly.
   friend class Integer;
 
+  RawBigint* BigArithmeticOp(Token::Kind operation, const Bigint& other) const;
+
   static RawBigint* New(const String& str, Heap::Space space = Heap::kNew);
 
   // Returns a canonical Bigint object allocated in the old gen space.