Strong mode fix for json_map_test

The maps returned by JSON.decode have reified type Map<dynamic,
dynamic>, so when we iterate through them using forEach, we need to
pass in a callback that doesn't expect the key type to be String.

Change-Id: I93bb13705916c668ab6fc207ffbf4f50554ee979
Reviewed-on: https://dart-review.googlesource.com/19443
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

tests/corelib_2/json_map_test.dart

@@ -18,7 +18,7 @@ Map jsonify(Map map) {
 
 List listEach(Map map) {
   var result = [];
-  map.forEach((String key, value) {
+  map.forEach((key, value) {
     result.add(key);
     result.add(value);
   });