Fix List.map.join with no separator.

sdk/lib/_collection_dev/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._collection.dev;
 
 typedef T _Transformation<S, T>(S value);
 
 class MappedIterable<S, T> extends Iterable<T> {
   final Iterable<S> _iterable;
@@ skipping 261 matching lines @@
 
   String join([String separator]) {
     int start = _startIndex;
     int end = _endIndex;
     if (start == end) return "";
     StringBuffer buffer = new StringBuffer("${_f(_list[start])}");
     if (_list.length != length) {
       throw new ConcurrentModificationError(_list);
     }
     for (int i = start + 1; i < end; i++) {
-      buffer.add(separator);
+      if (separator != null && separator != "") {
+        buffer.add(separator);
+      }
       buffer.add("${_f(_list[i])}");
       if (_list.length != length) {
         throw new ConcurrentModificationError(_list);
       }
     }
     return buffer.toString();
   }
 
   Iterable<T> where(bool test(T element)) => super.where(test);
 
@@ skipping 381 matching lines @@
 
   Set toSet() => new Set<E>();
 }
 
 /** The always empty iterator. */
 class EmptyIterator<E> implements Iterator<E> {
   const EmptyIterator();
   bool moveNext() => false;
   E get current => null;
 }