Add static ensure() methods to unmodifiable views.

sdk/lib/collection/maps.dart

Index: sdk/lib/collection/maps.dart
diff --git a/sdk/lib/collection/maps.dart b/sdk/lib/collection/maps.dart
index 7945e7ea643411074c5e65769a5870c0733913c7..06dde937cf4a362483200ce9d54904f9738de074 100644
--- a/sdk/lib/collection/maps.dart
+++ b/sdk/lib/collection/maps.dart
@@ -208,8 +208,21 @@ class MapView<K, V> implements Map<K, V> {
  * 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