Temporarily bring in code from the async package.

lib/src/util/forkable_stream.dart

Index: lib/src/util/forkable_stream.dart
diff --git a/lib/src/util/forkable_stream.dart b/lib/src/util/forkable_stream.dart
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+// Copyright (c) 2015, 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.
+
+// TODO(nweiz): Get rid of this when https://codereview.chromium.org/1241723003/
+// lands.
+library test.util.forkable_stream;
+
+import 'dart:async';
+
+import 'package:async/async.dart' hide ForkableStream;
+
+/// A single-subscription stream from which other streams may be forked off at
+/// the current position.
+///
+/// This adds an operation, [fork], which produces a new stream that
+/// independently emits the same events as this stream. Unlike the branches
+/// produced by [StreamSplitter], a fork only emits events that arrive *after*
+/// the call to [fork].
+///
+/// Each fork can be paused or canceled independently of one another and of this
+/// stream. The underlying stream will be listened to once any branch is
+/// listened to. It will be paused when all branches are paused or not yet
+/// listened to. It will be canceled when all branches have been listened to and
+/// then canceled.
+class ForkableStream<T> extends StreamView<T> {
+  /// The underlying stream.
+  final Stream _sourceStream;
+
+  /// The subscription to [_sourceStream].
+  ///
+  /// This will be `null` until this stream or any of its forks are listened to.
+  StreamSubscription _subscription;
+
+  /// Whether this has been cancelled and no more forks may be created.
+  bool _isCanceled = false;
+
+  /// The controllers for any branches that have not yet been canceled.
+  ///
+  /// This includes a controller for this stream, until that has been cancelled.
+  final _controllers = new Set<StreamController<T>>();
+
+  /// Creates a new forkable stream wrapping [sourceStream].
+  ForkableStream(Stream sourceStream)
+      // Use a completer here so that we can provide its stream to the
+      // superclass constructor while also adding the stream controller to
+      // [_controllers].
+      : this._(sourceStream, new StreamCompleter());
+
+  ForkableStream._(this._sourceStream, StreamCompleter completer)
+      : super(completer.stream) {
+    completer.setSourceStream(_fork(primary: true));
+  }
+
+  /// Creates a new fork of this stream.
+  ///
+  /// From this point forward, the fork will emit the same events as this
+  /// stream. It will *not* emit any events that have already been emitted by
+  /// this stream. The fork is independent of this stream, which means each one
+  /// may be paused or canceled without affecting the other.
+  ///
+  /// If this stream is done or its subscription has been canceled, this returns
+  /// an empty stream.
+  Stream<T> fork() => _fork(primary: false);
+
+  /// Creates a stream forwarding [_sourceStream].
+  ///
+  /// If [primary] is true, this is the stream underlying this object;
+  /// otherwise, it's a fork. The only difference is that when the primary
+  /// stream is canceled, [fork] starts throwing [StateError]s.
+  Stream<T> _fork({bool primary: false}) {
+    if (_isCanceled) {
+      var controller = new StreamController<T>()..close();
+      return controller.stream;
+    }
+
+    var controller;
+    controller = new StreamController<T>(
+        onListen: () => _onListenOrResume(controller),
+        onCancel: () => _onCancel(controller, primary: primary),
+        onPause: () => _onPause(controller),
+        onResume: () => _onListenOrResume(controller),
+        sync: true);
+
+    _controllers.add(controller);
+
+    return controller.stream;
+  }
+
+  /// The callback called when `onListen` or `onResume` is called for the branch
+  /// managed by [controller].
+  ///
+  /// This ensures that we're subscribed to [_sourceStream] and that the
+  /// subscription isn't paused.
+  void _onListenOrResume(StreamController<T> controller) {
+    if (controller.isClosed) return;
+    if (_subscription == null) {
+      _subscription =
+          _sourceStream.listen(_onData, onError: _onError, onDone: _onDone);
+    } else {
+      _subscription.resume();
+    }
+  }
+
+  /// The callback called when `onCancel` is called for the branch managed by
+  /// [controller].
+  ///
+  /// This cancels or pauses the underlying subscription as necessary. If
+  /// [primary] is true, it also ensures that future calls to [fork] throw
+  /// [StateError]s.
+  Future _onCancel(StreamController<T> controller, {bool primary: false}) {
+    if (primary) _isCanceled = true;
+
+    if (controller.isClosed) return null;
+    _controllers.remove(controller);
+
+    if (_controllers.isEmpty) return _subscription.cancel();
+
+    _onPause(controller);
+    return null;
+  }
+
+  /// The callback called when `onPause` is called for the branch managed by
+  /// [controller].
+  ///
+  /// This pauses the underlying subscription if necessary.
+  void _onPause(StreamController<T> controller) {
+    if (controller.isClosed) return;
+    if (_subscription.isPaused) return;
+    if (_controllers.any((controller) =>
+        controller.hasListener && !controller.isPaused)) {
+      return;
+    }
+
+    _subscription.pause();
+  }
+
+  /// Forwards data events to all branches.
+  void _onData(value) {
+    // Don't iterate directly over the set because [controller.add] might cause
+    // it to be modified synchronously.
+    for (var controller in _controllers.toList()) {
+      controller.add(value);
+    }
+  }
+
+  /// Forwards error events to all branches.
+  void _onError(error, StackTrace stackTrace) {
+    // Don't iterate directly over the set because [controller.addError] might
+    // cause it to be modified synchronously.
+    for (var controller in _controllers.toList()) {
+      controller.addError(error, stackTrace);
+    }
+  }
+
+  /// Forwards close events to all branches.
+  void _onDone() {
+    _isCanceled = true;
+
+    // Don't iterate directly over the set because [controller.close] might
+    // cause it to be modified synchronously.
+    for (var controller in _controllers.toList()) {
+      controller.close();
+    }
+    _controllers.clear();
+  }
+}
+