The setRange methods works on non-extensible arrays. Document it as such.

corelib/src/list.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.
 
 /**
  * A [List] is an indexable collection with a length. It can be of
  * fixed size or extendable.
  */
 interface List<E> extends Collection<E> default ListFactory<E> {
 
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    * Returns an empty list if [length] is 0.
    * Throws an [IllegalArgumentException] if [length] is negative.
    * Throws an [IndexOutOfRangeException] if [start] or
    * [:start + length:] are out of range.
    */
   List<E> getRange(int start, int length);
 
   /**
    * Copies [length] elements of the [from] array, starting
    * from [startFrom], into [:this:], starting at [start].
-   * Throws an [UnsupportedOperationException] if the list is
-   * not extendable.
    * If [length] is 0, this method does not do anything.
    * Throws an [IllegalArgumentException] if [length] is negative.
    * Throws an [IndexOutOfRangeException] if [start] or
    * [:start + length:] are out of range for [:this:], or if
    * [startFrom] is out of range for [from].
    */
   void setRange(int start, int length, List<E> from, [int startFrom]);
 
   /**
    * Removes the range in the list starting from [start] to
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    * not extendable.
    * If [length] is 0, this method does not do anything.
    * If [start] is the length of the array, this method inserts the
    * range at the end of the array.
    * Throws an [IllegalArgumentException] if [length] is negative.
    * Throws an [IndexOutOfRangeException] if [start] or
    * [:start + length:] are out of range.
    */
   void insertRange(int start, int length, [E initialValue]);
 }