Make default argument to Iterable.join be "".

runtime/lib/growable_array.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.
 
 class _GrowableObjectArray<T> implements List<T> {
   factory _GrowableObjectArray._uninstantiable() {
     throw new UnsupportedError(
         "GrowableObjectArray can only be allocated by the VM");
   }
 
@@ skipping 224 matching lines @@
   }
 
   void forEach(f(T element)) {
     // TODO(srdjan): Use IterableMixinWorkaround.forEach(this, f);
     // Accessing the list directly improves DeltaBlue performance by 25%.
     for (int i = 0; i < length; i++) {
       f(this[i]);
     }
   }
 
-  String join([String separator]) {
+  String join([String separator = ""]) {
     if (isEmpty) return "";
     if (this.length == 1) return "${this[0]}";
     StringBuffer buffer = new StringBuffer();
-    if (separator == null || separator == "") {
+    if (separator.isEmpty) {
       for (int i = 0; i < this.length; i++) {
         buffer.write("${this[i]}");
       }
     } else {
       buffer.write("${this[0]}");
       for (int i = 1; i < this.length; i++) {
         buffer.write(separator);
         buffer.write("${this[i]}");
       }
     }
@@ skipping 87 matching lines @@
   }
 
   Set<T> toSet() {
     return new Set<T>.from(this);
   }
 
   Map<int, T> asMap() {
     return IterableMixinWorkaround.asMapList(this);
   }
 }