Fix error reported by analyzer.

lib/src/protobuf/field_set.dart

Index: lib/src/protobuf/field_set.dart
diff --git a/lib/src/protobuf/field_set.dart b/lib/src/protobuf/field_set.dart
index 6e99fe7197f9140c74a23735dc2aec2add1c31ee..e089108beee6c9d0df9305b1558df2c9d1f93c7a 100644
--- a/lib/src/protobuf/field_set.dart
+++ b/lib/src/protobuf/field_set.dart
@@ -361,7 +361,7 @@ class _FieldSet {
     int hash;
 
     void hashEnumList(PbList enums) {
-      for (final enm in enums) {
+      for (ProtobufEnum enm in enums) {
         hash = (31 * hash + enm.value) & 0x3fffffff;
       }
     }