Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/codegen_helper.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.
 
 import 'dart:async';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/options.dart' show CompilerOptions;
 import 'package:compiler/src/null_compiler_output.dart';
 import 'memory_source_file_helper.dart';
 
@@ skipping 13 matching lines @@
       handler,
       new CompilerOptions.parse(
           entryPoint: uri,
           libraryRoot: libraryRoot,
           packageRoot: packageRoot,
           options: options));
   return compiler.run(uri).then((success) {
     Expect.isTrue(success);
     Map<String, String> result = new Map<String, String>();
     var backend = compiler.backend;
-    for (var element in backend.generatedCode.keys) {
+    for (dynamic element in backend.generatedCode.keys) {
       if (element.compilationUnit.script.readableUri != uri) continue;
       var name = element.name;
       var code = backend.getGeneratedCode(element);
       result[name] = code;
     }
     return result;
   });
 }