Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/list_tracer3_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.
 
 // We used to always nullify the element type of a list we are tracing in
 // the presence of a fixed length list constructor call.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
-import 'package:compiler/src/types/types.dart' show ContainerTypeMask, TypeMask;
+import 'package:compiler/src/types/types.dart' show ContainerTypeMask;
 
 import 'compiler_helper.dart';
 import 'type_mask_test_helper.dart';
 
 const String TEST = r'''
 var myList = [];
 var otherList = ['foo', 42];
 main() {
   var a = otherList[0];
   a += 54;
@@ skipping 12 matching lines @@
           var element = findElement(compiler, name);
           ContainerTypeMask mask = typesInferrer.getTypeOfElement(element);
           Expect.equals(type, simplify(mask.elementType, closedWorld), name);
         }
 
         var interceptorType =
             typesInferrer.closedWorld.commonMasks.interceptorType;
         checkType('myList', interceptorType);
       }));
 }