Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/flatten_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 flatten_test;
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'type_test_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;
 
 void main() {
   asyncTest(() => TypeEnvironment.create(r"""
       abstract class F<T> implements Future<T> {}
       abstract class G<T> implements Future<G<T>> {}
       abstract class H<T> implements Future<H<H<T>>> {}
       """).then((env) {
         void check(
             ResolutionDartType T, ResolutionDartType expectedFlattenedType) {
           ResolutionDartType flattenedType = env.flatten(T);
@@ skipping 52 matching lines @@
         // flatten(Future<F<int>>) = int
         check(instantiate(Future_, [F_int]), int_);
 
         // flatten(Future<G<int>>) = int
         check(instantiate(Future_, [G_int]), G_int);
 
         // flatten(Future<H<int>>) = int
         check(instantiate(Future_, [H_int]), H_H_int);
       }));
 }