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

tests/html/queryall_test.dart

Index: tests/html/queryall_test.dart
diff --git a/tests/html/queryall_test.dart b/tests/html/queryall_test.dart
index e75ddfbdff1a8349d428fb7edbdd491df3dc93ec..6b701573ac002a32f6cd1ec3e13868cd97fef29a 100644
--- a/tests/html/queryall_test.dart
+++ b/tests/html/queryall_test.dart
@@ -47,6 +47,13 @@ main() {
       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);