Add StreamChannel.withGuarantees and StreamChannelController.

lib/src/stream_channel_controller.dart

Index: lib/src/stream_channel_controller.dart
diff --git a/lib/src/stream_channel_controller.dart b/lib/src/stream_channel_controller.dart
new file mode 100644
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+++ b/lib/src/stream_channel_controller.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:async';
+
+import '../stream_channel.dart';
+
+/// A controller for exposing a new [StreamChannel].
+///
+/// This exposes two connected [StreamChannel]s, [local] and [foreign]. The
+/// user's code should use [local] to emit and receive events. Then [foreign]
+/// can be returned for others to use. For example, here's a simplified version
+/// of the implementation of [new IsolateChannel]:
+///
+/// ```dart
+/// StreamChannel isolateChannel(ReceivePort receivePort, SendPort sendPort) {
+///   var controller = new StreamChannelController();
+///
+///   // Pipe all events from the receive port into the local sink...
+///   receivePort.pipe(controller.local.sink);
+///
+///   // ...and all events from the local stream into the send port.
+///   controller.local.listen(sendPort.add, onDone: receivePort.close);
+///
+///   // Then return the foreign controller for your users to use.
+///   return controller.foreign;
+/// }
+/// ```
+class StreamChannelController<T> {
+  /// The local channel.
+  ///
+  /// This channel should be used directly by the creator of this
+  /// [StreamChannelController] to send and receive events.
+  StreamChannel<T> get local => _local;
+  StreamChannel<T> _local;
+
+  /// The foreign channel.
+  ///
+  /// This channel should be returned to external users so they can communicate
+  /// with [local].
+  StreamChannel<T> get foreign => _foreign;
+  StreamChannel<T> _foreign;
+
+  /// Creates a [StreamChannelController].
+  ///
+  /// If [sync] is true, events added to either channel's sink are synchronously
+  /// dispatched to the other channel's stream. This should only be done if the
+  /// source of those events is already asynchronous.
+  StreamChannelController({bool sync: false}) {
+    var localToForeignController = new StreamController<T>(sync: sync);
+    var foreignToLocalController = new StreamController<T>(sync: sync);
+    _local = new StreamChannel<T>.withGuarantees(
+        foreignToLocalController.stream, localToForeignController.sink);
+    _foreign = new StreamChannel<T>.withGuarantees(
+        localToForeignController.stream, foreignToLocalController.sink);
+  }
+}