Change List.sort to not have a default parameter value.

samples/swarm/swarm_ui_lib/observable/observable.dart

 // Copyright (c) 2011, 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.
 
 library observable;
 
 part 'ChangeEvent.dart';
 part 'EventBatch.dart';
 
 /**
@@ skipping 140 matching lines @@
   void set length(int value) {
     _internal.length = value;
     recordGlobalChange();
   }
 
   void clear() {
     _internal.clear();
     recordGlobalChange();
   }
 
-  void sort([Comparator compare = Comparable.compare]) {
+  void sort([int compare(var a, var b)]) {
+    if (compare == null) compare = Comparable.compare;
     _internal.sort(compare);
     recordGlobalChange();
   }
 
   void add(T element) {
     recordListInsert(length, element);
     _internal.add(element);
   }
 
   void addLast(T element) {
@@ skipping 114 matching lines @@
     // Only fire on an actual change.
     if (!identical(newValue, _value)) {
       final oldValue = _value;
       _value = newValue;
       recordPropertyUpdate("value", newValue, oldValue);
     }
   }
 
   T _value;
 }