Make default argument to Iterable.join be "".

sdk/lib/core/iterable.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.
 
 part of dart.core;
 
 /**
  * The [Iterable] interface allows to get an [Iterator] out of an
  * [Iterable] object.
  *
@@ skipping 128 matching lines @@
     return true;
   }
 
   /**
    * Convert each element to a [String] and concatenate the strings.
    *
    * Converts each element to a [String] by calling [Object.toString] on it.
    * Then concatenates the strings, optionally separated by the [separator]
    * string.
    */
-  String join([String separator]) {
+  String join([String separator = ""]) {
     StringBuffer buffer = new StringBuffer();
-    buffer.writeAll(this, separator == null ? "" : separator);
+    buffer.writeAll(this, separator);
     return buffer.toString();
   }
 
   /**
    * Returns true if one element of this collection satisfies the
    * predicate [f]. Returns false otherwise.
    */
   bool any(bool f(E element)) {
     for (E element in this) {
       if (f(element)) return true;
@@ skipping 284 matching lines @@
  */
 abstract class BidirectionalIterator<T> extends Iterator<T> {
   /**
    * Move back to the previous element.
    *
    * Returns true and updates [current] if successful. Returns false
    * and sets [current] to null if there is no previous element.
    */
   bool movePrevious();
 }