Add List.unmodifiable constructor.

runtime/lib/array_patch.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // The _GrowableArrayMarker class is used to signal to the List() factory
 // whether a parameter was passed.
 class _GrowableArrayMarker implements int {
   const _GrowableArrayMarker();
 }
 
@@ skipping 30 matching lines @@
       return list;
     }
     List<E> list = new _GrowableList<E>(0);
     for (E e in elements) {
       list.add(e);
     }
     if (growable) return list;
     return makeListFixedLength(list);
   }
 
+  /* patch */ factory List.unmodifiable(Iterable elements) {
+    List result = new List<E>.from(elements, growable: false);
+    return makeFixedListUnmodifiable(result);
+  }
+
   // Factory constructing a mutable List from a parser generated List literal.
   // [elements] contains elements that are already type checked.
   factory List._fromLiteral(List elements) {
     if (elements.isEmpty) {
       return new _GrowableList<E>(0);
     }
     var result = new _GrowableList<E>.withData(elements);
     result._setLength(elements.length);
     return result;
   }
 }