Make mappedBy lazy.

tests/html/element_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 ElementTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_individual_config.dart';
 import 'dart:html';
 import 'dart:svg';
 
@@ skipping 626 matching lines @@
 
     test('forEach', () {
       var els = [];
       getQueryAll().forEach((el) => els.add(el));
       expect(els[0], isAnchorElement);
       expect(els[1], isSpanElement);
       expect(els[2], isHRElement);
     });
 
     test('map', () {
-      var texts = getQueryAll().mappedBy((el) => el.text);
+      var texts = getQueryAll().mappedBy((el) => el.text).toList();
       expect(texts, equals(['Dart!', 'Hello', '']));
     });
 
     test('filter', () {
       var filtered = getQueryAll().where((n) => n is SpanElement);
       expect(filtered.length, 1);
       expect(filtered[0], isSpanElement);
       expect(filtered, isElementList);
     });
 
@@ skipping 77 matching lines @@
     });
 
     test('getRange', () {
       var range = makeElList().getRange(1, 2);
       expect(range, isElementList);
       expect(range[0], isImageElement);
       expect(range[1], isInputElement);
     });
   });
 }