meta: Add TargetKind.enumValue

In
[the spec](https://github.com/dart-lang/language/blob/main/accepted/2.17/enhanced-enums/feature-specification.md),
these are referred to as "values", so I went with that wording here. In
the analyzer, I found one case where we call them the "constants"
(EnumDeclaration.constants). I'd prefer to stick with the more common
term, "values."

Fixes https://github.com/dart-lang/sdk/issues/47312

Change-Id: I3ea4650fdeb15d3d23fb7ca42bcf6e0ec0e511f7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/360661
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

pkg/meta/CHANGELOG.md

@@ -1,11 +1,13 @@
 ## 1.14.0-wip
 
 - Introduce `TargetKind.constructor`, to indicate that an annotation is valid on
-  any constructor.
+  any constructor declaration.
 - Introduce `TargetKind.directive`, to indicate that an annotation is valid on
   any directive.
+- Introduce `TargetKind.enumValue`, to indicate that an annotation is valid on
+  any enum value declaration.
 - Introduce `TargetKind.typeParamteer`, to indicate that an annotation is valid
-  on any type parameter.
+  on any type parameter declaration.
 - Introduce `@doNotSubmit` to annotate members that should not be submitted to
   source control, typically because they are intended to be used ephemerally
   during development.

pkg/meta/lib/meta_meta.dart

@@ -53,6 +53,9 @@ class TargetKind {
   /// `part of` directive.
   static const directive = TargetKind._('directives', 'directive');
 
+  /// Indicates that an annotation is valid on any enum value declaration.
+  static const enumValue = TargetKind._('enum values', 'enumValue');
+
   /// Indicates that an annotation is valid on any enum declaration.
   static const enumType = TargetKind._('enums', 'enumType');
 
@@ -125,6 +128,7 @@ class TargetKind {
     classType,
     constructor,
     directive,
+    enumValue,
     enumType,
     extension,
     extensionType,