[expect] Replace @alwaysThrows with Never

Actually, for `Expect.fail`, the return type is kept as `void`, as
changing this to `Never` would apparently change a lot of static
analysis of language and co19 tests. It seems the `@alwaysThrows`
annotation is no longer (or was never) relied upon for static
analysis (usually around dead code analysis).

Bug: https://github.com/dart-lang/sdk/issues/49583
Change-Id: I6d9daaa1d290ab0322a529faaf9574fe83b9cd25
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/279742
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

pkg/expect/lib/expect.dart

@@ -6,8 +6,6 @@
 /// for simple unit-tests.
 library expect;
 
-import 'package:meta/meta.dart';
-
 /// Whether the program is running without sound null safety.
 bool get hasUnsoundNullSafety => const <Null>[] is List<Object>;
 
@@ -275,7 +273,9 @@ class Expect {
   }
 
   // Unconditional failure.
-  @alwaysThrows
+  // This function always throws, as [_fail] always throws.
+  // TODO(srawlins): It would be more correct to change the return type to
+  // `Never`, which would require refactoring many language and co19 tests.
   static void fail(String msg) {
     _fail("Expect.fail('$msg')");
   }
@@ -693,7 +693,6 @@ class Expect {
   static String _getMessage(String reason) =>
       (reason.isEmpty) ? "" : ", '$reason'";
 
-  @alwaysThrows
   static Never _fail(String message) {
     throw ExpectException(message);
   }

pkg/expect/pubspec.yaml

@@ -13,7 +13,6 @@ environment:
 
 # Use 'any' constraints here; we get our versions from the DEPS file.
 dependencies:
-  meta: any
   smith: any
 
 # Use 'any' constraints here; we get our versions from the DEPS file.