Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/subtype_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 subtype_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() {
   testInterfaceSubtype();
   testCallableSubtype();
   testFunctionSubtyping();
   testTypedefSubtyping();
   testFunctionSubtypingOptional();
   testTypedefSubtypingOptional();
   testFunctionSubtypingNamed();
   testTypedefSubtypingNamed();
@@ skipping 763 matching lines @@
         expect(false, J_U, num_);
         expect(false, J_U, int_);
         expect(false, J_U, String_);
         expect(true, J_U, dynamic_);
         expect(false, J_U, J_T);
         expect(true, J_U, J_S);
         expect(true, J_U, J_U);
         expect(false, J_U, A_T);
       }));
 }