Generate contains() method for list mixin only if not already defined.

tests/html/queryall_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('NodeListTest');
 #import('../../pkg/unittest/unittest.dart');
 #import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
 
 main() {
@@ skipping 29 matching lines @@
     });
 
   test('queryAll-canvas', () {
       List<CanvasElement> all = queryAll('canvas');
       for (var e in all) {
         expect(e is CanvasElement, isTrue);
       }
       expect(all.length, equals(2));
     });
 
+  test('queryAll-contains', () {
+      List<Element> all = queryAll('*');
+      for (var e in all) {
+        expect(all.contains(e), isTrue);
+      }
+    });
+
   test('queryAll-filter', () {
       List<Element> all = queryAll('*');
       List<CanvasElement> canvases = all.filter((e) => e is CanvasElement);
       for (var e in canvases) {
         expect(e is CanvasElement, isTrue);
       }
       expect(canvases.length, equals(2));
     });
 
   test('node.queryAll', () {
       List<Element> list = div.queryAll('*');
       expect(list.length, equals(5));
       expect(list[0] is DivElement, isTrue);
       expect(list[1] is CanvasElement, isTrue);
       expect(list[2] is DivElement, isTrue);
       expect(list[3] is DivElement, isTrue);
       expect(list[4] is CanvasElement, isTrue);
     });
 
   test('immutable', () {
       List<Element> list = div.queryAll('*');
       int len = list.length;
       expect(() { list.add(new DivElement()); }, throwsException);
       expect(list.length, equals(len));
     });
 }