Make mappedBy lazy.

tests/corelib/list_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.
 
 class ListTest {
 
   static testMain() {
     testList();
     testExpandableList();
   }
 
   static void expectValues(list, val1, val2, val3, val4) {
     Expect.equals(true, list.length == 4);
     Expect.equals(true, list.length == 4);
     Expect.equals(true, !list.isEmpty);
     Expect.equals(list[0], val1);
     Expect.equals(list[1], val2);
     Expect.equals(list[2], val3);
     Expect.equals(list[3], val4);
   }
 
   static void testClosures(List list) {
     testMap(val) {return val * 2 + 10; }
-    Collection mapped = list.mappedBy(testMap);
+    List mapped = list.mappedBy(testMap).toList();
     Expect.equals(mapped.length, list.length);
     for (var i = 0; i < list.length; i++) {
       Expect.equals(mapped[i], list[i]*2 + 10);
     }
 
     testFilter(val) { return val == 3; }
     Collection filtered = list.where(testFilter);
     Expect.equals(filtered.length, 1);
 
     testEvery(val) { return val != 11; }
@@ skipping 108 matching lines @@
   }
 }
 
 class Yes {
   operator ==(var other) => true;
 }
 
 main() {
   ListTest.testMain();
 }