Added a `ResultFuture` class.

CHANGELOG.md

 ## 1.3.0
 
 - Added `StreamCompleter` class for creating a stream now and providing its
   events later as another stream.
 
 - Added `StreamQueue` class which allows requesting events from a stream
   before they are avilable. It is like a `StreamIterator` that can queue
   requests.
 
 - Added `SubscriptionStream` which creates a single-subscription stream
   from an existing stream subscription.
 
+- Added a `ResultFuture` class for synchronously accessing the result of a
+  wrapped future.
+
 - Added `FutureGroup.onIdle` and `FutureGroup.isIdle`, which provide visibility
   into whether a group is actively waiting on any futures.
 
 - Added delegating wrapper classes for a number of core async types:
   `DelegatingFuture`, `DelegatingStreamConsumer`, `DelegatingStreamController`,
   `DelegatingSink`, `DelegatingEventSink`, `DelegatingStreamSink`, and
   `DelegatingStreamSubscription`. These are all simple wrappers that forward all
   calls to the wrapped objects. They can be used to expose only the desired
   interface for subclasses, or extended to add extra functionality.
 
 ## 1.2.0
 
 - Added a `FutureGroup` class for waiting for a group of futures, potentially of
   unknown size, to complete.
 
 - Added a `StreamGroup` class for merging the events of a group of streams,
   potentially of unknown size.
 
 ## 1.1.1
 
 - Updated SDK version constraint to at least 1.9.0.
 
 ## 1.1.0
 
 - ChangeLog starts here.