Introduce @Sealed annotation

This annotation will act as an easy way for users to experiment with a language
feature under consideration, called "sealed classes."

Bug: #27372
Change-Id: Ieb8bc70edaf8c11c41f0f47c01951e8311736c1f
Reviewed-on: https://dart-review.googlesource.com/69601
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Lasse R.H. Nielsen <[email protected]>
Reviewed-by: Erik Ernst <[email protected]>
Reviewed-by: Leaf Petersen <[email protected]>

Author: srawlins

pkg/meta/CHANGELOG.md

@@ -1,3 +1,9 @@
+## 1.1.7
+
+* Introduce `@sealed` to declare that a class cannot be extended by, implemented
+  by, or mixed into another class. (SDK issue
+  [27372](https://github.com/dart-lang/sdk/issues/27372)).
+
 ## 1.1.6
 
 * Set max SDK version to <3.0.0.

pkg/meta/lib/meta.dart

@@ -188,6 +188,18 @@ const _Protected protected = const _Protected();
 ///   corresponding to a named parameter that has this annotation.
 const Required required = const Required();
 
+/// Annotation marking a class as unusable as a super-type, outside of the
+/// package in which the class is declared.
+///
+/// Given a class _C_, which is annotated with this, and a class _D_, which
+/// extends, implements, or mixes in _C_, then tools, such as the analyzer, may
+/// emit warnings if:
+///
+/// * _C_ and _D_ are declared in different packages, or
+/// * _C_ and _D_ are declared in the same package, and _D_ is not also
+/// annotated with this.
+const _Sealed sealed = const _Sealed();
+
 /// Used to annotate a field that is allowed to be overridden in Strong Mode.
 const _Virtual virtual = const _Virtual();
 
@@ -284,6 +296,10 @@ class _Protected {
   const _Protected();
 }
 
+class _Sealed {
+  const _Sealed();
+}
+
 class _Virtual {
   const _Virtual();
 }

pkg/meta/pubspec.yaml

@@ -1,5 +1,5 @@
 name: meta
-version: 1.1.6
+version: 1.1.7
 author: Dart Team <[email protected]>
 homepage: https://github.com/dart-lang/sdk/tree/master/pkg/meta
 description: >