Revert "Add "contains" method to Collection."

lib/core/list.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.
 
 /**
  * A [List] is an indexable collection with a length. It can be of
  * fixed size or extendable.
  */
-interface List<E> extends Collection<E>, Sequence<E>
-                  default ListImplementation<E> {
+interface List<E> extends Collection<E> default ListImplementation<E> {
+
   /**
    * Creates a list of the given [length].
    *
    * If no [length] argument is supplied an extendable list of
    * length 0 is created.
    *
    * If a [length] argument is supplied, a fixed size list of that
    * length is created.
    */
   List([int length]);
@@ skipping 146 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 [ArgumentError] 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]);
 }