Change generic comment syntax to real generic syntax.

lib/src/future_group.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:async';
 
 /// A collection of futures waits until all added [Future]s complete.
 ///
 /// Futures are added to the group with [add]. Once you're finished adding
 /// futures, signal that by calling [close]. Then, once all added futures have
@@ skipping 49 matching lines @@
     if (_closed) throw new StateError("The FutureGroup is closed.");
 
     // Ensure that future values are put into [values] in the same order they're
     // added to the group by pre-allocating a slot for them and recording its
     // index.
     var index = _values.length;
     _values.add(null);
 
     _pending++;
     task.then((value) {
-      if (_completer.isCompleted) return;
+      if (_completer.isCompleted) return null;
 
       _pending--;
       _values[index] = value;
 
-      if (_pending != 0) return;
+      if (_pending != 0) return null;
       if (_onIdleController != null) _onIdleController.add(null);
 
-      if (!_closed) return;
+      if (!_closed) return null;
       if (_onIdleController != null) _onIdleController.close();
       _completer.complete(_values);
     }).catchError((error, stackTrace) {
-      if (_completer.isCompleted) return;
+      if (_completer.isCompleted) return null;
       _completer.completeError(error, stackTrace);
     });
   }
 
   /// Signals to the group that the caller is done adding futures, and so
   /// [future] should fire once all added futures have completed.
   void close() {
     _closed = true;
     if (_pending != 0) return;
     if (_completer.isCompleted) return;
     _completer.complete(_values);
   }
 }