Add FutureGroup.onIdle and FutureGroup.isIdle.

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 `DelegatingStreamSubscription` which is a simple wrapper around
   a `StreamSubscription` that forwards all call to the wrapped subscription.
   It can be extended to wrap extra functionality around a subscription.
 
 - Added `SubscriptionStream` which creates a single-subscription stream
   from an existing stream subscription.
 
+- Added `FutureGroup.onIdle` and `FutureGroup.isIdle`, which provide visibility
+  into whether a group is actively waiting on any futures.
+
 ## 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.