fixes #27586, prefer context type in generic inference

tests/lib_strong/convert/json_chunk_test.dart

 // Copyright (c) 2014, 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.
 
 library json_test;
 
 import "package:expect/expect.dart";
 import "dart:convert";
 
 bool badFormat(e) => e is FormatException;
 
 jsonTest(testName, expect, action(sink)) {
-  var sink = new ChunkedConversionSink.withCallback((values) {
+  var sink = new ChunkedConversionSink<dynamic>.withCallback((values) {
     var value = values[0];
     Expect.equals(expect, value, "$testName:$value");
   });
   var decoderSink = JSON.decoder.startChunkedConversion(sink);
   action(decoderSink);
 }
 
 jsonThrowsTest(testName, action(sink)) {
-  var sink = new ChunkedConversionSink.withCallback((values) {
+  var sink = new ChunkedConversionSink<dynamic>.withCallback((values) {
     Expect.fail("Should have thrown: $testName");
   });
   var decoderSink = JSON.decoder.startChunkedConversion(sink);
   Expect.throws(() { action(decoderSink); }, (e) => e is FormatException,
                 testName);
 }
 
 main() {
   testNumbers();
   testStrings();
@@ skipping 131 matching lines @@
         jsonTest("$s1|$s2a|$s2b", expected, (sink) {
           sink.add(s1);
           sink.add(s2a);
           sink.add(s2b);
           sink.close();
         });
       }
     }
   }
 }