All strong mode cleaning of dart2js.

tests/compiler/dart2js/analyze_helper.dart

Index: tests/compiler/dart2js/analyze_helper.dart
diff --git a/tests/compiler/dart2js/analyze_helper.dart b/tests/compiler/dart2js/analyze_helper.dart
index 6c83521dcde9b57068feda151b1e0d2ac13d262d..fa06084ce6a8a0ccc9dbdd69d6cb298f7fef19e1 100644
--- a/tests/compiler/dart2js/analyze_helper.dart
+++ b/tests/compiler/dart2js/analyze_helper.dart
@@ -132,8 +132,8 @@ class CollectingDiagnosticHandler extends FormattingDiagnosticHandler {
   }
 
   @override
-  void report(Message message, Uri uri, int begin, int end, String text,
-      api.Diagnostic kind) {
+  void report(covariant Message message, Uri uri, int begin, int end,
+      String text, api.Diagnostic kind) {
     if (kind == api.Diagnostic.WARNING) {
       if (checkWhiteList(uri, message, text)) {
         // Suppress whitelisted warnings.