Deprecate alwaysThrows

Fixes #49583

Change-Id: Icc10dbba642703098708202537c733ff1d48b9db
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/279651
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Phil Quitslund <pquitslund@google.com>

Author: srawlins

pkg/meta/CHANGELOG.md

@@ -4,6 +4,8 @@
   extend classes marked `base`, `final` or `interface`.
 * Introduce `@MustBeOverridden` to annotate class or mixin members which must be
   overridden in all subclasses.
+* Deprecate `@alwaysThrows`, which can be replaced by using a return type of
+  'Never'.
 
 ## 1.8.0
 

pkg/meta/lib/meta.dart

@@ -51,6 +51,7 @@ import 'meta_meta.dart';
 /// Tools, such as the analyzer, can also expect this contract to be enforced;
 /// that is, tools may emit warnings if a function with this annotation
 /// _doesn't_ always throw.
+@Deprecated("Use a return type of 'Never' instead")
 const _AlwaysThrows alwaysThrows = _AlwaysThrows();
 
 /// Used to annotate a parameter of an instance method that overrides another