Update `@factory` to allow for statics and nulls.

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.
 ///
 /// 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.
+/// Used to annotate an instance or static method `m`. Indicates that `m` must
+/// either be abstract or must return a newly allocated object or `null`. 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.
+/// * the annotation is associated with a method that has this annotation that
+///   can return anything other than a newly allocated object or null.

[Brian Wilkerson, 2016/06/21 22:28:15]

"null" --> "`null`"

Also, restore a variation on the initial restriction:

* the annotation is associated with anything other than a method,

[pquitslund, 2016/06/21 22:33:36]

Done.

 const _Factory factory = const _Factory();
 
 /// Used to annotate a const constructor `c`. Indicates that any invocation of
 /// the constructor must use the keyword `const` unless one or more of the
 /// arguments to the constructor is not a compile-time constant.
 ///
 /// Tools, such as the analyzer, can provide feedback if
 ///
 /// * the annotation is associated with anything other than a const constructor,
 ///   or
@@ skipping 87 matching lines @@
   const _MustCallSuper();
 }
 
 class _OptionalTypeArgs {
   const _OptionalTypeArgs();
 }
 
 class _Protected {
   const _Protected();
 }