Add Future.timeout.

pkg/barback/lib/src/cancelable_future.dart

Index: pkg/barback/lib/src/cancelable_future.dart
diff --git a/pkg/barback/lib/src/cancelable_future.dart b/pkg/barback/lib/src/cancelable_future.dart
index 284df411362d8dbcfd0af110d37c960a4f7b5625..4454f5cf2b383f143f082121dc0ce668d743123a 100644
--- a/pkg/barback/lib/src/cancelable_future.dart
+++ b/pkg/barback/lib/src/cancelable_future.dart
@@ -32,7 +32,8 @@ class CancelableFuture<T> implements Future<T> {
   Future then(onValue(T value), {Function onError}) =>
     _completer.future.then(onValue, onError: onError);
   Future<T> whenComplete(action()) => _completer.future.whenComplete(action);
-
+  Future timeout(Duration timeLimit, [void onTimeout()]) =>
+    _completer.future.timeout(timeLimit, onTimeout);
   /// Cancels this future.
   void cancel() {
     _canceled = true;