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

runtime/vm/bigint_operations.h

Index: runtime/vm/bigint_operations.h
===================================================================
--- runtime/vm/bigint_operations.h	(revision 22880)
+++ runtime/vm/bigint_operations.h	(working copy)
@@ -62,8 +62,8 @@
   static bool FitsIntoSmi(const Bigint& bigint);
   static RawSmi* ToSmi(const Bigint& bigint);
 
-  static bool FitsIntoMint(const Bigint& bigint);
-  static int64_t ToMint(const Bigint& bigint);
+  static bool FitsIntoInt64(const Bigint& bigint);
+  static int64_t ToInt64(const Bigint& bigint);
 
   static bool FitsIntoUint64(const Bigint& bigint);
   static bool AbsFitsIntoUint64(const Bigint& bigint);