Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/jsinterop/world_test.dart

Index: tests/compiler/dart2js/jsinterop/world_test.dart
diff --git a/tests/compiler/dart2js/jsinterop/world_test.dart b/tests/compiler/dart2js/jsinterop/world_test.dart
index 6b24ecb6062d69c3569fc52b108b1c486a8d35ad..821b25c06956fd709d5367c1d3aaaf29c428d06b 100644
--- a/tests/compiler/dart2js/jsinterop/world_test.dart
+++ b/tests/compiler/dart2js/jsinterop/world_test.dart
@@ -6,10 +6,8 @@ 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';
@@ -90,7 +88,6 @@ $mainSource
     }
 
     ClosedWorld world = env.closedWorld;
-    JavaScriptBackend backend = env.compiler.backend;
     ClassElement Object_ = registerClass(world.commonElements.objectClass);
     ClassElement Interceptor =
         registerClass(world.commonElements.jsInterceptorClass);