Ignore a few 'unused' values in the _FilePermission enum (dart-lang/io#54)

pkgs/io/lib/src/permissions.dart

diff --git a/pkgs/io/lib/src/permissions.dart b/pkgs/io/lib/src/permissions.dart
index 789d327..e890373 100644
--- a/pkgs/io/lib/src/permissions.dart
+++ b/pkgs/io/lib/src/permissions.dart
@@ -8,8 +8,10 @@
 /// What type of permission is granted to a file based on file permission roles.
 enum _FilePermission {
   execute,
-  write,
-  read,
+  // Although these two values are unused, their positions in the enum are
+  // meaningful.
+  write, // ignore: unused_field
+  read, // ignore: unused_field
   setGid,
   setUid,
   sticky,