Get rid of several old collection workarounds in serialization

pkg/serialization/lib/src/basic_rule.dart

 // Copyright (c) 2012, 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 serialization;
 
 // TODO(alanknight): Figure out how to reasonably separate out the things
 // that require reflection without making the API more awkward. Or if that is
 // in fact necessary. Maybe the tree-shaking will just remove it if unused.
 
@@ skipping 507 matching lines @@
   void invalidate() {
     _contents = null;
     contents;
   }
 
   Iterator<_Field> get iterator => contents.iterator;
 
   /** 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;
     }
     return _contents;
   }
 
   /** Iterate over the regular fields, i.e. those not used in the constructor.*/
   void forEachRegularField(Function f) {
     for (var each in contents) {
       if (each.isRegular) {
@@ skipping 131 matching lines @@
   if (value is String) {
     try {
       return new Symbol(value);
     } on ArgumentError {
       return null;
     };
   } else {
     return null;
   }
 }