fixes #27586, prefer context type in generic inference

tests/lib_strong/convert/json_chunk_test.dart

Index: tests/lib_strong/convert/json_chunk_test.dart
diff --git a/tests/lib_strong/convert/json_chunk_test.dart b/tests/lib_strong/convert/json_chunk_test.dart
index 28e92977913fe95ec8f44f3b2b1d8214d822a50b..87c4d9c80a5a8f41a926fe73bf813b9fad067a74 100644
--- a/tests/lib_strong/convert/json_chunk_test.dart
+++ b/tests/lib_strong/convert/json_chunk_test.dart
@@ -10,7 +10,7 @@ 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");
   });
@@ -19,7 +19,7 @@ jsonTest(testName, expect, action(sink)) {
 }
 
 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);