Add @factory 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 method `m`. Indicates that `m` must either be
+/// abstract or must return a newly allocated object. In addition, every method
+/// that either implements or overrides `m` is implicitly annotated with this
+/// same annotation.
+///
+/// Tools, such as the analyzer, can provide feedback if
+/// * the annotation is associated with anything other than an instance method,
+///   or
+/// * a method that has this annotation that can return anything other than a
+///   newly allocated object.
+const _Factory factory = const _Factory();
+
 /// Used to annotate an instance method `m`. Indicates that every invocation of
 /// a method that overrides `m` must also invoke `m`. In addition, every method
 /// that overrides `m` is implicitly annotated with this same annotation.
 ///
 /// Note that private methods with this annotation cannot be validly overridden
 /// outside of the library that defines the annotated method.
 ///
 /// Tools, such as the analyzer, can provide feedback if
 /// * the annotation is associated with anything other than an instance method,
 ///   or
@@ skipping 22 matching lines @@
 /// corresponding to `p`, despite the fact that `p` would otherwise be an
 /// optional parameter.
 ///
 /// 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 _Factory {
+  const _Factory();
+}
+
 class _MustCallSuper {
   const _MustCallSuper();
 }
 
 class _Protected {
   const _Protected();
 }
 
 class _Required {
   const _Required();
 }