Change new List(n) to return fixed length list.

tests/corelib/list_set_range_test.dart

 // Copyright (c) 2011, 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.
 
 main() {
   var list = [];
   list.setRange(0, 0, const []);
   list.setRange(0, 0, []);
   list.setRange(0, 0, const [], 1);
   list.setRange(0, 0, [], 1);
@@ skipping 64 matching lines @@
 
   // A zero length prevails, and does not throw an exception.
   list.setRange(-1, 0, [1]);
   Expect.listEquals([1, 2], list);
 
   list.setRange(0, 0, [1], -1);
   Expect.listEquals([1, 2], list);
 }
 
 void testNonExtendableList() {
-  var list = new List<int>.fixedLength(6);
+  var list = new List<int>(6);
   Expect.listEquals([null, null, null, null, null, null], list);
   list.setRange(0, 3, [1, 2, 3, 4]);
   list.setRange(3, 3, [1, 2, 3, 4]);
   Expect.listEquals([1, 2, 3, 1, 2, 3], list);
 }