Make default argument to Iterable.join be "".

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;
 
 import 'dart:collection';
 
 part 'ChangeEvent.dart';
 part 'EventBatch.dart';
@@ skipping 284 matching lines @@
 
   // Collection<T>:
   Iterable<T> where(bool f(T element)) => _internal.where(f);
   Iterable map(f(T element)) => _internal.map(f);
   Iterable expand(Iterable f(T element)) => _internal.expand(f);
   List<T> skip(int count) => _internal.skip(count);
   List<T> take(int count) => _internal.take(count);
   bool every(bool f(T element)) => _internal.every(f);
   bool any(bool f(T element)) => _internal.any(f);
   void forEach(void f(T element)) { _internal.forEach(f); }
-  String join([String separator]) => _internal.join(separator);
+  String join([String separator = ""]) => _internal.join(separator);
   T firstWhere(bool test(T value), {T orElse()}) {
     return _internal.firstWhere(test, orElse: orElse);
   }
   T lastWhere(bool test(T value), {T orElse()}) {
     return _internal.lastWhere(test, orElse: orElse);
   }
   T singleWhere(bool test(T value)) {
     return _internal.singleWhere(test);
   }
   T elementAt(int index) {
@@ skipping 23 matching lines @@
     // Only fire on an actual change.
     if (!identical(newValue, _value)) {
       final oldValue = _value;
       _value = newValue;
       recordPropertyUpdate("value", newValue, oldValue);
     }
   }
 
   T _value;
 }