Make an enumeration added for 1.9 a real enum

sdk/lib/io/file.dart

Index: sdk/lib/io/file.dart
diff --git a/sdk/lib/io/file.dart b/sdk/lib/io/file.dart
index 8a493a226a26d3401ff2c1023d2c3e83a9e263bb..ce7fa1b364b22dd57ce43e60c99a0db6c9d68a3d 100644
--- a/sdk/lib/io/file.dart
+++ b/sdk/lib/io/file.dart
@@ -34,16 +34,11 @@ const APPEND = FileMode.APPEND;
 
 
 /// Type of lock when requesting a lock on a file.
-class FileLock {
-  final int _lock;
-
+enum FileLock {
   /// Shared file lock.
-  static const SHARED = const FileLock._internal(0);
-
+  SHARED,
   /// Exclusive file lock.
-  static const EXCLUSIVE = const FileLock._internal(1);
-
-  const FileLock._internal(this._lock);
+  EXCLUSIVE
 }
 
 /**