Add the @protected annotation

pkg/meta/lib/meta.dart

 // Copyright (c) 2016, 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.
 
-/**
- * Constants for use in metadata annotations such as `@protected`.
- *
- * See also `@deprecated` and `@override` in the `dart:core` library.
- *
- * Annotations provide semantic information
- * that tools can use to provide a better user experience.
- * For example, an IDE might not autocomplete
- * the name of a function that's been marked `@deprecated`,
- * or it might display the function's name differently.
- *
- * For information on installing and importing this library, see the
- * [meta package on pub.dartlang.org]
- * (http://pub.dartlang.org/packages/meta).
- * For examples of using annotations, see
- * [Metadata](https://www.dartlang.org/docs/dart-up-and-running/ch02.html#metadata)
- * in the language tour.
- */
+/// Constants for use in metadata annotations such as `@protected`.
+///
+/// See also `@deprecated` and `@override` in the `dart:core` library.
+///
+/// Annotations provide semantic information that tools can use to provide a
+/// better user experience. For example, an IDE might not autocomplete the name
+/// of a function that's been marked `@deprecated`, or it might display the
+/// function's name differently.
+///
+/// For information on installing and importing this library, see the
+/// [meta package on pub.dartlang.org] (http://pub.dartlang.org/packages/meta).
+/// For examples of using annotations, see
+/// [Metadata](https://www.dartlang.org/docs/dart-up-and-running/ch02.html#metadata)
+/// in the language tour.
 library meta;
+
+/// Used to annotate an instance method `m` in a class `C`. Indicates that `m`
+/// should only be invoked from instance methods of `C` or classes that extend
+/// or mix in `C`, either directly or indirectly. Additionally indicates that
+/// `m` should only be invoked on `this`, whether explicitly or implicitly.
+///
+/// Tools, such as the analyzer, can provide feedback if an invocation of a
+/// method marked as being protected is used outside of an instance method
+/// defined on a class that extends or mixes in the class in which the protected
+/// method is defined, or that uses a receiver other than `this`.
+const _Protected protected = const _Protected();
+
+class _Protected {
+  const _Protected();
+}