Split dart:builtin into dart:builtin and dart:io.

runtime/bin/io_in.cc

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include <stdlib.h>
 #include <string.h>
 
 #include "bin/builtin.h"
 
-// The string on the next line will be filled in with the contents of the
-// builtin.dart file.
-// This string forms the content of builtin functionality which is injected
-// into standalone dart to provide some test/debug functionality.
-const char Builtin::Builtin_source_[] = {
+// The string on the next line will be filled in with the contents of
+// the dart files of the dart:io library.
+const char Builtin::io_source_[] = {
   {{DART_SOURCE}}
 };