Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/unused_empty_map_test.dart

 // Copyright (c) 2014, 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.
 
 // Ensure that unused empty HashMap nodes are dropped from the output.
 
 import 'package:expect/expect.dart';
 import 'package:compiler/compiler_new.dart';
 import 'package:async_helper/async_helper.dart';
 import 'memory_compiler.dart';
 
 const TEST_SOURCE = const {
   "main.dart": r"""
 void main() {
   var x = {};
   return;
 }
 """
 };
 
 const HASHMAP_EMPTY_CONSTRUCTOR = r"LinkedHashMap_LinkedHashMap$_empty";
 
 main() {
   asyncTest(() async {
     var collector = new OutputCollector();
-    var result = await runCompiler(
+    await runCompiler(
         memorySourceFiles: TEST_SOURCE, outputProvider: collector);
-    var compiler = result.compiler;
     String generated = collector.getOutput('', OutputType.js);
     Expect.isFalse(generated.contains(HASHMAP_EMPTY_CONSTRUCTOR));
   });
 }