Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/least_upper_bound_test.dart

 // Copyright (c) 2013, 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 least_upper_bound_test;
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/elements/resolution_types.dart';
-import 'package:compiler/src/elements/elements.dart' show Element, ClassElement;
+import 'package:compiler/src/elements/elements.dart' show ClassElement;
 
 import 'type_test_helper.dart';
 
 void main() {
   testInterface1();
   testInterface2();
   testGeneric();
   testMixin();
   testFunction();
   testTypeVariable();
@@ skipping 821 matching lines @@
         checkLub(X, B, B);
         checkLub(X, C, B);
         checkLub(X, S, Object_);
         checkLub(X, T, T);
         checkLub(X, U, B);
         checkLub(X, V, T);
         checkLub(X, W, T);
         checkLub(X, X, X);
       }));
 }