Rename exit_code file to match class name (dart-lang/io#11)

pkgs/io/lib/io.dart

diff --git a/pkgs/io/lib/io.dart b/pkgs/io/lib/io.dart
index 3af40ae..4e42aa9 100644
--- a/pkgs/io/lib/io.dart
+++ b/pkgs/io/lib/io.dart
@@ -2,7 +2,7 @@
 // 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.
 
-export 'src/exit_codes.dart' show ExitCode;
+export 'src/exit_code.dart' show ExitCode;
 export 'src/permissions.dart' show isExecutable;
 export 'src/process_manager.dart' show ProcessManager, Spawn;
 export 'src/shared_stdin.dart' show SharedStdIn, sharedStdIn;

pkgs/io/lib/src/exit_code.dart

diff --git a/pkgs/io/lib/src/exit_codes.dart b/pkgs/io/lib/src/exit_code.dart
similarity index 100%
rename from pkgs/io/lib/src/exit_codes.dart
rename to pkgs/io/lib/src/exit_code.dart