Make Transform a little more pleasant to use:

pkg/barback/lib/src/utils.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.
 
 library barback.utils;
 
 import 'dart:async';
 
 /// A pair of values.
 class Pair<E, F> {
@@ skipping 161 matching lines @@
   // We use a delayed future to allow runAsync events to finish. The
   // Future.value or Future() constructors use runAsync themselves and would
   // therefore not wait for runAsync callbacks that are scheduled after invoking
   // this method.
   return new Future.delayed(Duration.ZERO, () => pumpEventQueue(times - 1));
 }
 
 /// Like [new Future], but avoids issue 11911 by using [new Future.value] under
 /// the covers.
 Future newFuture(callback()) => new Future.value().then((_) => callback());
+
+/// Returns a buffered stream that will emit the same values as the stream
+/// returned by [future] once [future] completes. If [future] completes to an
+/// error, the return value will emit that error and then close.
+Stream futureStream(Future<Stream> future) {
+  var controller = new StreamController(sync: true);
+  future.then((stream) {
+    stream.listen(
+        controller.add,
+        onError: (error) => controller.addError(error),
+        onDone: controller.close);
+  }).catchError((e) {
+    controller.addError(e);
+    controller.close();
+  });
+  return controller.stream;
+}