Make mappedBy lazy.

tests/corelib/queue_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.
 
 library queue_test;
 
 class QueueTest {
 
   static testMain() {
     Queue queue = new Queue();
@@ skipping 23 matching lines @@
     checkQueue(queue, 3, 1110);
 
     int mapTest(int value) {
       return value ~/ 10;
     }
 
     bool is10(int value) {
       return (value == 10);
     }
 
-    Queue mapped = queue.mappedBy(mapTest);
+    Queue mapped = new Queue.from(queue.mappedBy(mapTest));
     checkQueue(mapped, 3, 111);
     checkQueue(queue, 3, 1110);
     Expect.equals(1, mapped.removeFirst());
     Expect.equals(100, mapped.removeLast());
     Expect.equals(10, mapped.removeFirst());
 
     Queue other = queue.where(is10);
     checkQueue(other, 1, 10);
 
     Expect.equals(true, queue.some(is10));
@@ skipping 122 matching lines @@
       entry1 = entry1.nextEntry();
       entry2 = entry2.nextEntry();
     }
     Expect.equals(null, entry2);
   }
 }
 
 main() {
   QueueTest.testMain();
 }