Fix error reported by analyzer.

lib/src/protobuf/field_set.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of protobuf;
 
 /// All the data in a GeneratedMessage.
 ///
 /// These fields and methods are in a separate class to avoid
 /// polymorphic access due to inheritance. This turns out to
@@ skipping 343 matching lines @@
   }
 
   /// Calculates a hash code based on the contents of the protobuf.
   ///
   /// The hash may change when any field changes (recursively).
   /// Therefore, protobufs used as map keys shouldn't be changed.
   int get _hashCode {
     int hash;
 
     void hashEnumList(PbList enums) {
-      for (final enm in enums) {
+      for (ProtobufEnum enm in enums) {
         hash = (31 * hash + enm.value) & 0x3fffffff;
       }
     }
 
     // Hashes the value of one field (recursively).
     void hashField(FieldInfo fi, value) {
       if (value is List && value.isEmpty) {
         return; // It's either repeated or an empty byte array.
       }
       hash = ((37 * hash) + fi.tagNumber) & 0x3fffffff;
@@ skipping 177 matching lines @@
   void _appendInvalidFields(List<String> problems, String prefix) {
     if (!_hasRequiredFields) return;
     for (var fi in _infos) {
       var value = _values[fi.index];
       fi._appendInvalidFields(problems, value, prefix);
     }
     // TODO(skybrian): search extensions as well
     // https://github.com/dart-lang/dart-protobuf/issues/46
   }
 }