Add unmodifiable list view.

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 18c1d7b73c02e56818d2f213cffe382095815990..06aa7677c73b96bd51a0f7e5d5a770ccd6e690a2 100644
--- a/sdk/lib/collection/collections.dart
+++ b/sdk/lib/collection/collections.dart
@@ -500,3 +500,17 @@ class Collections {
   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);
+}