Add static ensure() methods to unmodifiable views.

sdk/lib/collection/maps.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;
 
 /**
  * Base class for implementing a [Map].
  *
  * This class has a basic implementation of all but five of the members of
@@ skipping 190 matching lines @@
   Iterable<V> get values => _map.values;
 }
 
 /**
  * View of a [Map] that disallow modifying the map.
  *
  * A wrapper around a `Map` that forwards all members to the map provided in
  * the constructor, except for operations that modify the map.
  * Modifying operations throw instead.
  */
-class UnmodifiableMapView<K, V> =
-    MapView<K, V> with _UnmodifiableMapMixin<K, V>;
+class UnmodifiableMapView<K, V> extends MapView<K, V>
+    with _UnmodifiableMapMixin<K, V> {
+  UnmodifiableMapView(Map<K, V> map) : super(map);
+
+  /**
+   * Returns a view of [source] that's guaranteed to be unmodifiable.
+   *
+   * If [source] already extends [UnmodifiableMapBase] or [UnmodifiableMapView],
+   * it is returned without re-wrapping it.
+   */
+  static Map ensure(Map source) =>
+      source is _UnmodifiableMapMixin
+          ? source
+          : new UnmodifiableMapView(source);
+}
 
 /**
  * Helper class which implements complex [Map] operations
  * in term of basic ones ([Map.keys], [Map.operator []],
  * [Map.operator []=] and [Map.remove].)  Not all methods are
  * necessary to implement each particular operation.
  */
 class Maps {
   static bool containsValue(Map map, value) {
     for (final v in map.values) {
@@ skipping 121 matching lines @@
       map[keyIterator.current] = valueIterator.current;
       hasNextKey = keyIterator.moveNext();
       hasNextValue = valueIterator.moveNext();
     }
 
     if (hasNextKey || hasNextValue) {
       throw new ArgumentError("Iterables do not have same length.");
     }
   }
 }