Get rid of several old collection workarounds in serialization

pkg/serialization/lib/src/basic_rule.dart

Index: pkg/serialization/lib/src/basic_rule.dart
diff --git a/pkg/serialization/lib/src/basic_rule.dart b/pkg/serialization/lib/src/basic_rule.dart
index 005580a53f978b95e703111e6962bafb66bdd584..157410c1eb2fa8fed0d6662540a9992907ee264d 100644
--- a/pkg/serialization/lib/src/basic_rule.dart
+++ b/pkg/serialization/lib/src/basic_rule.dart
@@ -525,7 +525,7 @@ class _FieldList extends IterableBase<_Field> {
   /** Return a cached, sorted list of all the fields. */
   List<_Field> get contents {
     if (_contents == null) {
-      _contents = sorted(allFields.values);
+      _contents = allFields.values.toList()..sort();
       for (var i = 0; i < _contents.length; i++)
         _contents[i].index = i;
     }