Do not fail when an enum constant is annotated (issue 26413)

pkg/analyzer/lib/src/dart/constant/utilities.dart

Index: pkg/analyzer/lib/src/dart/constant/utilities.dart
diff --git a/pkg/analyzer/lib/src/dart/constant/utilities.dart b/pkg/analyzer/lib/src/dart/constant/utilities.dart
index 80e736bbb358cc939ba17b3dc3d57a8fbebb83c7..7815baca8d1c16a115b0b6ac729357c71bd2661f 100644
--- a/pkg/analyzer/lib/src/dart/constant/utilities.dart
+++ b/pkg/analyzer/lib/src/dart/constant/utilities.dart
@@ -172,8 +172,10 @@ class ConstantFinder extends RecursiveAstVisitor<Object> {
     super.visitAnnotation(node);
     ElementAnnotation elementAnnotation = node.elementAnnotation;
     if (elementAnnotation == null) {
-      // Analyzer ignores annotations on "part of" directives.
-      assert(node.parent is PartOfDirective);
+      // Analyzer ignores annotations on "part of" directives and on enum
+      // constant declarations.
+      assert(node.parent is PartOfDirective ||
+          node.parent is EnumConstantDeclaration);
     } else {
       constantsToCompute.add(elementAnnotation);
     }