Add List.unmodifiable constructor.

sdk/lib/core/list.dart

Index: sdk/lib/core/list.dart
diff --git a/sdk/lib/core/list.dart b/sdk/lib/core/list.dart
index 05c4c2e2b6d5df185d2ec015a501a82d55537ade..9d9a0933207b184c79efbc72b8b491d183ad87f7 100644
--- a/sdk/lib/core/list.dart
+++ b/sdk/lib/core/list.dart
@@ -122,6 +122,17 @@ abstract class List<E> implements Iterable<E>, EfficientLength {
   }
 
   /**
+   * Creates an unmodifiable list containing all [elements].
+   *
+   * The [Iterator] of [elements] provides the order of the elements.
+   *
+   * An unmodifiable list cannot have its length or elements changed.
+   * If the elements are themselves immutable, then the resulting list
+   * is also immutable.
+   */
+  external factory List.unmodifiable(Iterable elements);
+
+  /**
    * Returns the object at the given [index] in the list
    * or throws a [RangeError] if [index] is out of bounds.
    */