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

runtime/bin/io.dart

Index: runtime/bin/io.dart
diff --git a/runtime/bin/io.dart b/runtime/bin/io.dart
new file mode 100644
index 0000000000000000000000000000000000000000..f88e624860e1648a275e6951639a3a3d24860f2a
--- /dev/null
+++ b/runtime/bin/io.dart
@@ -0,0 +1,11 @@
+// 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.
+
+// The dart:io library is the concatenation of all the files in
+// io_sources.gypi. This file needs to be the first file in that
+// concatenation.
+
+#library("io");
+#import("dart:nativewrappers");
+#import("dart:coreimpl");