Add static ensure() methods to unmodifiable views.

sdk/lib/collection/collections.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;
 
 /**
  * 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> {
   final Iterable<E> _source;
+
   /** Create an unmodifiable list backed by [source]. */
   UnmodifiableListView(Iterable<E> source) : _source = source;
+
+  /**
+   * Returns a view of [source] that's guaranteed to be unmodifiable.
+   *
+   * If [source] already extends [UnmodifiableListBase] or
+   * [UnmodifiableListView], it is returned without re-wrapping it.
+   */
+  static List ensure(Iterable source) =>
+      source is UnmodifiableListBase
+          ? source
+          : new UnmodifiableListView(source);
+
   int get length => _source.length;
   E operator[](int index) => _source.elementAt(index);
 }