Add SingleSubscriptionTransformer.

lib/src/single_subscription_transformer.dart

Index: lib/src/single_subscription_transformer.dart
diff --git a/lib/src/single_subscription_transformer.dart b/lib/src/single_subscription_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.
+
+library async.single_subscription_transformer;
+
+import 'dart:async';
+
+/// A transformer that converts a broadcast stream into a single-subscription
+/// stream.
+///
+/// This buffers the broadcast stream's events, which means that it starts
+/// listening to a stream as soon as it's bound.
+class SingleSubscriptionTransformer<S, T> implements StreamTransformer<S, T> {
+  const SingleSubscriptionTransformer();
+
+  Stream<T> bind(Stream<S> stream) {
+    var subscription;
+    var controller = new StreamController(sync: true,
+        onCancel: () => subscription.cancel());
+    subscription = stream.listen(controller.add,
+        onError: controller.addError, onDone: controller.close);
+    return controller.stream;
+  }
+}