Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/jsinterop/world_test.dart

 // Copyright (c) 2016, 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 jsinterop.world_test;
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
-import 'package:compiler/src/common.dart';
 import 'package:compiler/src/elements/elements.dart' show ClassElement;
 import 'package:compiler/src/elements/names.dart';
-import 'package:compiler/src/js_backend/js_backend.dart';
 import 'package:compiler/src/universe/selector.dart';
 import 'package:compiler/src/world.dart';
 import '../type_test_helper.dart';
 
 void main() {
   asyncTest(() async {
     await testClasses();
   });
 }
 
@@ skipping 60 matching lines @@
 """,
         useMockCompiler: false);
     Map<String, ClassElement> classEnvironment = <String, ClassElement>{};
 
     ClassElement registerClass(ClassElement cls) {
       classEnvironment[cls.name] = cls;
       return cls;
     }
 
     ClosedWorld world = env.closedWorld;
-    JavaScriptBackend backend = env.compiler.backend;
     ClassElement Object_ = registerClass(world.commonElements.objectClass);
     ClassElement Interceptor =
         registerClass(world.commonElements.jsInterceptorClass);
     ClassElement JavaScriptObject =
         registerClass(world.commonElements.jsJavaScriptObjectClass);
     ClassElement A = registerClass(env.getElement('A'));
     ClassElement B = registerClass(env.getElement('B'));
     ClassElement C = registerClass(env.getElement('C'));
     ClassElement D = registerClass(env.getElement('D'));
     ClassElement E = registerClass(env.getElement('E'));
@@ skipping 94 matching lines @@
 
   await test('main() => newE();', directlyInstantiated: ['E']);
 
   await test('main() => newF();', directlyInstantiated: ['F']);
 
   await test('main() => [newD(), newE()];',
       directlyInstantiated: ['E'],
       abstractlyInstantiated: ['A', 'B', 'C', 'D'],
       indirectlyInstantiated: ['Object', 'Interceptor', 'JavaScriptObject']);
 }