Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/subtypeset_test.dart

 // Copyright (c) 2015, 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.
 
 // Test for iterators on for [SubclassNode].
 
 library subtypeset_test;
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'type_test_helper.dart';
-import 'package:compiler/src/elements/elements.dart' show Element, ClassElement;
+import 'package:compiler/src/elements/elements.dart' show ClassElement;
 import 'package:compiler/src/universe/class_set.dart';
 import 'package:compiler/src/world.dart';
 
 void main() {
   asyncTest(() => TypeEnvironment
           .create(
               r"""
       ///        A
       ///       / \
       ///      B   C
@@ skipping 51 matching lines @@
         checkClass(B, [B, D, E]);
         checkClass(C, [C, E, F, G, H, B, D, I]);
         checkClass(D, [D]);
         checkClass(E, [E]);
         checkClass(F, [F]);
         checkClass(G, [G]);
         checkClass(H, [H, I]);
         checkClass(I, [I]);
       }));
 }