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..8d68b5e4651a9497d863645bec66c401d15e5c78 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> {
+  /**
+   * Returns a view of [source] that's guaranteed to be unmodifiable.
+   *
+   * If [source] already extends [UnmodifiableMapBase] or [UnmodifiableMapView],
+   * this avoids re-wrapping it.

[Lasse Reichstein Nielsen, 2015/08/27 07:59:04]

Also say that it returns the original object unwrapped.

[nweiz, 2015/08/27 19:25:37]

Done.

+   */
+  static Map ensure(Map source) =>

[Lasse Reichstein Nielsen, 2015/08/27 07:59:04]

Move below constructor, and also don't like name here.

[nweiz, 2015/08/27 19:25:37]

Done.

+      source is _UnmodifiableMapMixin || source is ImmutableMap

[Lasse Reichstein Nielsen, 2015/08/27 07:59:04]

What is ImmutableMap? 
It looks like the VM's const map, but it doesn't exist in dart2js, so it will be
a malformed type (I don't remember if that means the test throws in checked mode
and succeeds in unchecked, or something else).

[nweiz, 2015/08/27 19:25:37]

Yes, this was intended to handle constant maps. I didn't realize it was VM-only,
though. Is there a reason constant Maps don't extend UnmodifiableMapBase but
constant lists do extend UnmodifiableListBase? Is there any other way we can
check for constant-ness?

+          ? source
+          : new UnmodifiableMapView(source);
+
+  UnmodifiableMapView(Map<K, V> map) : super(map);
+}
 
 /**
  * Helper class which implements complex [Map] operations