handle deserializing metadata diagnostic targets

Change-Id: I47f747baf48d6fc3a0e04871990d0743dbbb41e4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/352560
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Jake Macdonald <jakemac@google.com>
Auto-Submit: Jake Macdonald <jakemac@google.com>

pkg/_fe_analyzer_shared/lib/src/macros/executor/serialization_extensions.dart

@@ -501,9 +501,11 @@ extension DeserializerExtensions on Deserializer {
         new DiagnosticMessage(message, target: target.asDiagnosticTarget),
       TypeAnnotationImpl() =>
         new DiagnosticMessage(message, target: target.asDiagnosticTarget),
+      MetadataAnnotationImpl() =>
+        new DiagnosticMessage(message, target: target.asDiagnosticTarget),
       _ => throw new UnsupportedError(
-          'Unsupported target type ${target.runtimeType}, only Declarations '
-          'and TypeAnnotations are allowed.'),
+          'Unsupported target type ${target.runtimeType}, only Declarations, '
+          'TypeAnnotations, and Metadata are allowed.'),
     };
   }
 }

tests/language/macros/json/json_serializable_diagnostics_test.dart

@@ -85,7 +85,7 @@ class EnumField {
 }
 
 @JsonSerializable()
-class EnumField {
+class DuplicateJsonKey {
   @JsonKey()
   @JsonKey()
 //^^^^^^^^^^