Add list.removeAt method.

lib/core/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 ListImplementation<E> {
 
@@ skipping 78 matching lines @@
   int lastIndexOf(E element, [int start]);
 
   /**
    * Removes all elements in the list. The length of the list
    * becomes zero. Throws an [UnsupportedOperationException] if
    * the list is not extendable.
    */
   void clear();
 
   /**
+   * Removes the element at position[index] from the list.
+   *
+   * This reduces the length of the list by one and moves all later elements
+   * down by one position.
+   * Returns the removed element.
+   * Throws a [IllegalArgumentException] if [index] is not an integer, and
+   * [IndexOutOfRangeException] if the [index] does not point inside the list.
+   *
+   * Throws a [UnsupportedOperationException] if the list, or the length of
+   * the list, cannot be changed.
+   */
+  E removeAt(int index);
+
+  /**
    * Pops and returns the last element of the list.
    * Throws a [UnsupportedOperationException] if the length of the
    * list cannot be changed.
+   *
    */
   E removeLast();
 
   /**
    * Returns the last element of the list, or throws an out of bounds
    * exception if the list is empty.
    */
   E last();
 
   /**
@@ skipping 35 matching lines @@
    * not extendable.
    * If [length] is 0, this method does not do anything.
    * If [start] is the length of the list, this method inserts the
    * range at the end of the list.
    * Throws an [IllegalArgumentException] if [length] is negative.
    * Throws an [IndexOutOfRangeException] if [start] is negative or if
    * [start] is greater than the length of the list.
    */
   void insertRange(int start, int length, [E initialValue]);
 }