Add static ensure() methods to unmodifiable views.

tests/corelib/map_test.dart

Index: tests/corelib/map_test.dart
diff --git a/tests/corelib/map_test.dart b/tests/corelib/map_test.dart
index 79d1ace519b3fb3b0cfada9be45aefa09e513916..4898699902635af0956383616b95a377932e3e65 100644
--- a/tests/corelib/map_test.dart
+++ b/tests/corelib/map_test.dart
@@ -123,6 +123,7 @@ void main() {
   testUnmodifiableMap(const {1 : 37});
   testUnmodifiableMap(new UnmodifiableMapView({1 : 37}));
   testUnmodifiableMap(new UnmodifiableMapBaseMap([1, 37]));
+  testUnmodifiableMap(UnmodifiableMapView.ensure({1 : 37}));
 
   testFrom();
 }
@@ -750,6 +751,8 @@ void testUnmodifiableMap(Map map) {
   Expect.throws(() { map.remove(1); });
   Expect.throws(() { map[2] = 42; });
   Expect.throws(() { map.addAll({2 : 42}); });
+
+  Expect.isTrue(identical(map, UnmodifiableMapView.ensure(map)));
 }
 
 class Customer {