Make default argument to Iterable.join be "".

sdk/lib/collection/list.dart

 // Copyright (c) 2013, 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.collection;
 
 /**
  * Abstract implementation of a list.
  *
  * All operations are defined in terms of `length`, `operator[]`,
@@ skipping 163 matching lines @@
       if (compare(max, element) < 0) {
         max = element;
       }
       if (length != this.length) {
         throw new ConcurrentModificationError(this);
       }
     }
     return max;
   }
 
-  String join([String separator]) {
+  String join([String separator = ""]) {
     int length = this.length;
-    if (separator != null && !separator.isEmpty) {
+    if (!separator.isEmpty) {
       if (length == 0) return "";
       String first = "${this[0]}";
       if (length != this.length) {
         throw new ConcurrentModificationError(this);
       }
       StringBuffer buffer = new StringBuffer(first);
       for (int i = 1; i < length; i++) {
         buffer.write(separator);
-        buffer.write("${this[i]}");
+        buffer.write(this[i]);
         if (length != this.length) {
           throw new ConcurrentModificationError(this);
         }
       }
       return buffer.toString();
     } else {
       StringBuffer buffer = new StringBuffer();
       for (int i = 0; i < length; i++) {
-        buffer.write("${this[i]}");
+        buffer.write(this[i]);
         if (length != this.length) {
           throw new ConcurrentModificationError(this);
         }
       }
       return buffer.toString();
     }
   }
 
   Iterable<E> where(bool test(E element)) => new WhereIterable<E>(this, test);
 
@@ skipping 232 matching lines @@
     for (int i = startIndex; i >= 0; i--) {
       if (this[i] == element) {
         return i;
       }
     }
     return -1;
   }
 
   Iterable<E> get reversed => new ReversedListIterable(this);
 }