Small changes to make analyzer happy.

sdk/lib/_collection_dev/list.dart

 // Copyright (c) 2013, 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.
 
 part of dart._collection.dev;
 
 /**
  * Mixin that throws on the length changing operations of [List].
  *
  * Intended to mix-in on top of [ListMixin] for fixed-length lists.
@@ skipping 65 matching lines @@
   }
 }
 
 /**
  * Mixin for an unmodifiable [List] class.
  *
  * This overrides all mutating methods with methods that throw.
  * This mixin is intended to be mixed in on top of [ListMixin] on
  * unmodifiable lists.
  */
-abstract class UnmodifiableListMixin<E> {
+abstract class UnmodifiableListMixin<E> implements List<E> {
 
   void operator []=(int index, E value) {
     throw new UnsupportedError(
         "Cannot modify an unmodifiable list");
   }
 
   void set length(int newLength) {
     throw new UnsupportedError(
         "Cannot change the length of an unmodifiable list");
   }
@@ skipping 157 matching lines @@
 }
 
 class ReversedListIterable<E> extends ListIterable<E> {
   Iterable<E> _source;
   ReversedListIterable(this._source);
 
   int get length => _source.length;
 
   E elementAt(int index) => _source.elementAt(_source.length - 1 - index);
 }