fixes #27586, prefer context type in generic inference

tests/lib_strong/convert/json_test.dart

Index: tests/lib_strong/convert/json_test.dart
diff --git a/tests/lib_strong/convert/json_test.dart b/tests/lib_strong/convert/json_test.dart
index ef7b077d4c8c622a5f59044de9fd3b9c8073236f..1f2f4650c8b2642059d2b909da9c58c099d5ee00 100644
--- a/tests/lib_strong/convert/json_test.dart
+++ b/tests/lib_strong/convert/json_test.dart
@@ -38,7 +38,7 @@ void testJson(json, expected) {
   for (var reviver in [null, (k, v) => v]) {
     for (var split in [0, 1, 2, 3]) {
       var name = (reviver == null) ? "" : "reviver:";
-      var sink = new ChunkedConversionSink.withCallback((values) {
+      var sink = new ChunkedConversionSink<dynamic>.withCallback((values) {
         var value = values[0];
         compare(expected, value, "$name$value");
       });