Add the @required 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.
 library meta;
 
 /// Used to annotate an instance member (method, getter, setter, operator, or
 /// field) `m` in a class `C`. If the annotation is on a field it applies to the
 /// getter and setter, if appropriate, implied by the field. 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
-/// member marked as being protected is used outside of an instance member
-/// defined on a class that extends or mixes in the class in which the protected
-/// member is defined, or that uses a receiver other than `this`.
+/// Tools, such as the analyzer, can provide feedback if
+/// * the annotation is associated with anything other than an instance member,
+///   or
+/// * an invocation of a member marked as being protected is used outside of an
+///   instance member defined on a class that extends or mixes in the class in
+///   which the protected member is defined, or that uses a receiver other than
+///   `this`.
+const _Protected protected = const _Protected();
+
+/// Used to annotate a named parameter `p` in a method or function `f`.
+/// Indicates that every invocation of `f` must include an argument
+/// corresponding to `p`, despite the fact that `p` would otherwise be an
+/// optional parameter.
 ///
-/// Tools can also provide feedback if the annotation is used on anything other
-/// than an instance member.
-const _Protected protected = const _Protected();
+/// Tools, such as the analyzer, can provide feedback if
+/// * the annotation is associated with anything other than a named parameter,
+///   or
+/// * an invocation of a method or function does not include an argument
+///   corresponding to a named parameter that has this annotation.
+const _Required required = const _Required();
 
 class _Protected {
   const _Protected();
 }
+
+class _Required {
+  const _Required();
+}