Add Map.unmodifiable constructor.

sdk/lib/core/map.dart

Index: sdk/lib/core/map.dart
diff --git a/sdk/lib/core/map.dart b/sdk/lib/core/map.dart
index c011a2044a5ade009120ebda2ec5379815b91210..d19d1089c1623ebf43704012fba7b5baaa205a05 100644
--- a/sdk/lib/core/map.dart
+++ b/sdk/lib/core/map.dart
@@ -45,12 +45,28 @@ abstract class Map<K, V> {
    * The [other] map itself can have any type.
    *
    * A `LinkedHashMap` requires the keys to implement compatible
-   * `operator==` and `hashCode`, and it allows null as a key.
+   * `operator==` and `hashCode`, and it allows `null` as a key.
    * It iterates in key insertion order.
    */
   factory Map.from(Map other) = LinkedHashMap<K, V>.from;
 
   /**
+   * Creates an unmodifiable hash based map containing the entries of [other].
+   *
+   * The keys must all be assignable to [K] and the values to [V].
+   * The [other] map itself can have any type.
+   *
+   * The map requires the keys to implement compatible
+   * `operator==` and `hashCode`, and it allows `null` as a key.
+   * The created map iterates keys in a fixed order,
+   * preserving the order provided by [other].
+   *
+   * The resulting map behaves like the result of [Map.from],
+   * except that the map returned by this constructor is not modifiable.
+   */
+  external factory Map.unmodifiable(Map other);
+
+  /**
    * Creates an identity map with the default implementation, [LinkedHashMap].
    *
    * The returned map allows `null` as a key.