Add a document-by-document JSON transformer.

lib/src/json_document_transformer.dart

Index: lib/src/json_document_transformer.dart
diff --git a/lib/src/json_document_transformer.dart b/lib/src/json_document_transformer.dart
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+// Copyright (c) 2016, 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 'dart:convert';
+
+import 'package:async/async.dart';
+
+import '../stream_channel.dart';
+import 'stream_channel_transformer.dart';
+
+/// The canonical instance of [JsonDocumentTransformer].
+final jsonDocument = new JsonDocumentTransformer();
+
+/// A [StreamChannelTransformer] that transforms JSON documents—strings that
+/// contain individual objects encoded as JSON—into decoded Dart objects.
+///
+/// This decodes JSON that's emitted by the transformed channel's stream, and
+/// encodes objects so that JSON is passed to the transformed channel's sink.
+class JsonDocumentTransformer
+    implements StreamChannelTransformer<String, Object> {
+  /// The underlying codec that implements the encoding and decoding logic.
+  final JsonCodec _codec;
+
+  /// Creates a new transformer.
+  ///
+  /// The [reviver] and [toEncodable] arguments work the same way as the
+  /// corresponding arguments to [new JsonCodec].
+  JsonDocumentTransformer({reviver(key, value), toEncodable(object)})
+      : _codec = new JsonCodec(reviver: reviver, toEncodable: toEncodable);
+
+  JsonDocumentTransformer._(this._codec);
+
+  StreamChannel bind(StreamChannel<String> channel) {
+    var stream = channel.stream.map(_codec.decode);
+    var sink = new StreamSinkTransformer.fromHandlers(handleData: (data, sink) {
+      sink.add(_codec.encode(data));
+    }).bind(channel.sink);
+    return new StreamChannel(stream, sink);
+  }
+}