Add transformer support to Converters.

tests/lib/convert/streamed_conversion_json_utf8_encode_test.dart

+// Copyright (c) 2013, 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.
+
+import "package:expect/expect.dart";
+import 'dart:async';
+import 'dart:convert';
+import 'json_unicode_tests.dart';
+import '../../async_helper.dart';
+
+final JSON_UTF8 = JSON.fuse(UTF8);
+
+
+Stream<List<int>> encode(Object o) {
+  var controller;
+  controller = new StreamController(onListen: () {
+    controller.add(o);
+    controller.close();
+  });
+  return controller.stream.transform(JSON_UTF8.encoder);
+}
+
+void testUnpaused(List<int> expected, Stream stream) {
+  asyncStart();
+  stream.toList().then((list) {
+    var combined = [];
+    list.forEach(combined.addAll);
+    Expect.listEquals(expected, combined);
+    asyncEnd();
+  });
+}
+
+void testWithPauses(List<int> expected, Stream stream) {
+  asyncStart();
+  var accumulated = [];
+  var sub;
+  sub = stream.listen((x) {
+    accumulated.addAll(x);
+    sub.pause(new Future.delayed(Duration.ZERO));
+  }, onDone: () {
+    Expect.listEquals(expected, accumulated);
+    asyncEnd();
+  });
+}
+
+void main() {
+  for (var test in JSON_UNICODE_TESTS) {
+    var expected = test[0];
+    var object = test[1];
+    testUnpaused(expected, encode(object));
+    testWithPauses(expected, encode(object));
+  }
+}