Add unmodifiable list view.

sdk/lib/collection/collections.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.
 
 part of dart.collection;
 
 /**
  * This class provides default implementations for Iterables (including Lists).
  *
  * Once Dart receives Mixins it will be replaced with mixin classes.
@@ skipping 482 matching lines @@
       => IterableMixinWorkaround.skipList(list, n);
 
   /** Deprecated. Use the same method in [IterableMixinWorkaround] instead.*/
   @deprecated
   static Iterable skipWhile(Iterable iterable, bool test(var value))
       => IterableMixinWorkaround.skipWhile(iterable, test);
 
   static String collectionToString(Collection c)
       => ToString.collectionToString(c);
 }
+
+/**
+ * An unmodifiable [List] view of another List.
+ *
+ * The source of the elements may be a [List] or any [Iterable] with
+ * efficient [Iterable.length] and [Iterable.elementAt].
+ */
+class UnmodifiableListView<E> extends UnmodifiableListBase<E> {
+  Iterable<E> _source;
+  /** Create an unmodifiable list backed by [source]. */
+  UnmodifiableListView(Iterable<E> source) : _source = source;
+  int get length => _source.length;
+  E operator[](int index) => _source.elementAt(index);
+}