fixes #27586, prefer context type in generic inference

tests/lib_strong/convert/json_test.dart

 // Copyright (c) 2012, 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.
 // Disable background compilation so that the Issue 24908 can be reproduced.
 // VMOptions=--no-background-compilation
 
 library json_test;
 
 import "package:expect/expect.dart";
 import "dart:convert";
@@ skipping 20 matching lines @@
       Expect.isTrue(expected.compareTo(actual) == 0,
                     "$path: Expected: $expected, was: $actual");
     } else {
       // String, bool, null.
       Expect.equals(expected, actual, path);
     }
   }
   for (var reviver in [null, (k, v) => v]) {
     for (var split in [0, 1, 2, 3]) {
       var name = (reviver == null) ? "" : "reviver:";
-      var sink = new ChunkedConversionSink.withCallback((values) {
+      var sink = new ChunkedConversionSink<dynamic>.withCallback((values) {
         var value = values[0];
         compare(expected, value, "$name$value");
       });
       var decoderSink = JSON.decoder.startChunkedConversion(sink);
       switch (split) {
         case 0:
           // Split after first char.
           decoderSink.add(json.substring(0, 1));
           decoderSink.add(json.substring(1));
           decoderSink.close();
@@ skipping 267 matching lines @@
 }
 
 main() {
   testNumbers();
   testStrings();
   testWords();
   testObjects();
   testArrays();
   testWhitespace();
 }