Make mappedBy lazy.

tests/corelib/set_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 SetTest {
 
   static testMain() {
     Set set = new Set();
     Expect.equals(0, set.length);
     set.add(1);
@@ skipping 33 matching lines @@
     Expect.equals(10 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1, sum);
 
     Expect.equals(true, set.isSubsetOf(set));
     Expect.equals(true, set.containsAll(set));
 
     // Test Set.map.
     testMap(int val) {
       return val * val;
     }
 
-    Set mapped = set.mappedBy(testMap);
+    Set mapped = set.mappedBy(testMap).toSet();
     Expect.equals(10, mapped.length);
 
     Expect.equals(true, mapped.contains(0));
     Expect.equals(true, mapped.contains(1));
     Expect.equals(true, mapped.contains(4));
     Expect.equals(true, mapped.contains(9));
     Expect.equals(true, mapped.contains(16));
     Expect.equals(true, mapped.contains(25));
     Expect.equals(true, mapped.contains(36));
     Expect.equals(true, mapped.contains(49));
     Expect.equals(true, mapped.contains(64));
     Expect.equals(true, mapped.contains(81));
 
     sum = 0;
     set.forEach(testForEach);
     Expect.equals(10 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1, sum);
 
     sum = 0;
 
     mapped.forEach(testForEach);
     Expect.equals(1 + 2 + 5 + 10 + 17 + 26 + 37 + 50 + 65 + 82, sum);
 
     // Test Set.filter.
     testFilter(int val) {
       return val.isEven;
     }
 
-    Set filtered = set.where(testFilter);
+    Set filtered = set.where(testFilter).toSet();
 
     Expect.equals(5, filtered.length);
 
     Expect.equals(true, filtered.contains(0));
     Expect.equals(true, filtered.contains(2));
     Expect.equals(true, filtered.contains(4));
     Expect.equals(true, filtered.contains(6));
     Expect.equals(true, filtered.contains(8));
 
     sum = 0;
@@ skipping 69 matching lines @@
     set.clear();
     Expect.equals(0, set.length);
     set.add(11);
     Expect.equals(1, set.length);
   }
 }
 
 main() {
   SetTest.testMain();
 }