Add "contains" method to Collection.

lib/html/src/_Lists.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.
 
 class _Lists {
-
   /**
    * Returns the index in the array [a] of the given [element], starting
    * the search at index [startIndex] to [endIndex] (exclusive).
    * Returns -1 if [element] is not found.
    */
   static int indexOf(List a,
                      Object element,
                      int startIndex,
                      int endIndex) {
     if (startIndex >= a.length) {
@@ skipping 42 matching lines @@
     if (length < 0) throw new ArgumentError('length');
     if (start < 0) throw new IndexOutOfRangeException(start);
     int end = start + length;
     if (end > a.length) throw new IndexOutOfRangeException(end);
     for (int i = start; i < end; i++) {
       accumulator.add(a[i]);
     }
     return accumulator;
   }
 }