Let list-classes call Collections.joinList (which avoids allocating an iterator).

runtime/lib/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.
 
 
 // TODO(srdjan): Use shared array implementation.
 class _ObjectArray<E> implements List<E> {
 
   factory _ObjectArray(int length) native "ObjectArray_allocate";
 
@@ skipping 52 matching lines @@
 
   bool contains(E element) {
     return Collections.contains(this, element);
   }
 
   void forEach(f(E element)) {
     Collections.forEach(this, f);
   }
 
   String join([String separator]) {
-    return Collections.join(this, separator);
+    return Collections.joinList(this, separator);
   }
 
   List mappedBy(f(E element)) {
     return new MappedList<E, dynamic>(this, f);
   }
 
   reduce(initialValue, combine(previousValue, E element)) {
     return Collections.reduce(this, initialValue, combine);
   }
 
@@ skipping 187 matching lines @@
 
   void forEach(f(E element)) {
     Collections.forEach(this, f);
   }
 
   List mappedBy(f(E element)) {
     return new MappedList<E, dynamic>(this, f);
   }
 
   String join([String separator]) {
-    return Collections.join(this, separator);
+    return Collections.joinList(this, separator);
   }
 
   reduce(initialValue, combine(previousValue, E element)) {
     return Collections.reduce(this, initialValue, combine);
   }
 
   Iterable<E> where(bool f(E element)) {
     return new WhereIterable<E>(this, f);
   }
 
@@ skipping 143 matching lines @@
     }
     _position = _length;
     _current = null;
     return false;
   }
 
   E get current {
     return _current;
   }
 }