Make 'where' lazy.

tests/html/xmlelement_test.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.
 
 library XMLElementTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 main() {
@@ skipping 97 matching lines @@
     });
 
     test('iterator', () {
       final classes = <String>[];
       for (var el in makeClassSet()) {
         classes.add(el);
       }
       expect(classes, unorderedEquals(['foo', 'bar', 'baz']));
     });
 
-    test('map', () {
+    test('mappedBy', () {
       expect(makeClassSet().mappedBy((c) => c.toUpperCase()).toList(),
           unorderedEquals(['FOO', 'BAR', 'BAZ']));
     });
 
-    test('filter', () {
-      expect(makeClassSet().where((c) => c.contains('a')),
+    test('where', () {
+      expect(makeClassSet().where((c) => c.contains('a')).toSet(),
           unorderedEquals(['bar', 'baz']));
     });
 
     test('every', () {
       expect(makeClassSet().every((c) => c is String), isTrue);
       expect(makeClassSet().every((c) => c.contains('a')), isFalse);
     });
 
     test('some', () {
       expect(makeClassSet().some((c) => c.contains('a')), isTrue);
@@ skipping 438 matching lines @@
     makeElement().rect.then(
       expectAsync1(ElementRect rect) {
         expectEmptyRect(rect.client);
         expectEmptyRect(rect.offset);
         expectEmptyRect(rect.scroll);
         expectEmptyRect(rect.bounding);
         expect(rect.clientRects.isEmpty, isTrue);
       }));
   });
 }