Add @visibleForTesting annotation to meta.

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
@@ skipping 79 matching lines @@
 /// Tools, such as the analyzer, can provide feedback if
 ///
 /// * the annotation is associated with anything other than a named parameter,
 /// * the annotation is associated with a named parameter in a method `m1` that
 ///   overrides a method `m0` and `m0` defines a named parameter with the same
 ///   name that does not have this annotation, 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();
 
+/// Used to annotate a declaration was made public, so that it is more visible
+/// than otherwise necessary, to make code testable.
+///
+/// Tools, such as the analyzer, can provide feedback if
+///
+/// * the annotation is associated with a declaration not in the `lib` folder
+///   of a package;
+///   or
+/// * the declaration is referenced outside of its the defining library or a
+///   library which is in the `test` folder of the defining package.
+const _VisibleForTesting visibleForTesting = const _VisibleForTesting();
+
 /// Used to annotate a named parameter `p` in a method or function `f`.
 ///
 /// See [required] for more details.
 class Required {
   /// A human-readable explanation of the reason why the annotated parameter is
   /// required. For example, the annotation might look like:
   ///
   ///     ButtonWidget({
   ///         Function onHover,
   ///         @Required('Buttons must do something when pressed')
@@ skipping 18 matching lines @@
   const _MustCallSuper();
 }
 
 class _OptionalTypeArgs {
   const _OptionalTypeArgs();
 }
 
 class _Protected {
   const _Protected();
 }
+
+class _VisibleForTesting {
+  const _VisibleForTesting();
+}