Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/codegen_helper.dart

Index: tests/compiler/dart2js/codegen_helper.dart
diff --git a/tests/compiler/dart2js/codegen_helper.dart b/tests/compiler/dart2js/codegen_helper.dart
index b578c567589a5c9bc0aec7c31d25c32eb10a7129..a5f37bcb44d284c03b4a5c220673904fbd2e4003 100644
--- a/tests/compiler/dart2js/codegen_helper.dart
+++ b/tests/compiler/dart2js/codegen_helper.dart
@@ -31,7 +31,7 @@ Future<Map<String, String>> generate(String code,
     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);