Add static ensure() methods to unmodifiable views.

sdk/lib/collection/collections.dart

Index: sdk/lib/collection/collections.dart
diff --git a/sdk/lib/collection/collections.dart b/sdk/lib/collection/collections.dart
index fb9a94b8832dcbfb685b08ca6d39ab0750444a8e..e8dd28847935e11328a5b9a611ed2b3c540b4c9e 100644
--- a/sdk/lib/collection/collections.dart
+++ b/sdk/lib/collection/collections.dart
@@ -12,8 +12,21 @@ part of dart.collection;
  */
 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);
 }