Use FutureOr in Completer.complete.

sdk/lib/async/future_impl.dart

 // Copyright (c) 2012, 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.
 
 part of dart.async;
 
 /** The onValue and onError handlers return either a value or a future */
 typedef FutureOr<T> _FutureOnValue<S, T>(S value);
 /** Test used by [Future.catchError] to handle skip some errors. */
 typedef bool _FutureErrorTest(var error);
 /** Used by [WhenFuture]. */
 typedef _FutureAction();
 
 abstract class _Completer<T> implements Completer<T> {
   final _Future<T> future = new _Future<T>();
 
-  void complete([value]);
+  void complete([FutureOr<T> value]);
 
   void completeError(Object error, [StackTrace stackTrace]) {
     error = _nonNullError(error);
     if (!future._mayComplete) throw new StateError("Future already completed");
     AsyncError replacement = Zone.current.errorCallback(error, stackTrace);
     if (replacement != null) {
       error = _nonNullError(replacement.error);
       stackTrace = replacement.stackTrace;
     }
     _completeError(error, stackTrace);
   }
 
   void _completeError(Object error, StackTrace stackTrace);
 
   // The future's _isComplete doesn't take into account pending completions.
   // We therefore use _mayComplete.
   bool get isCompleted => !future._mayComplete;
 }
 
 class _AsyncCompleter<T> extends _Completer<T> {
 
-  void complete([value]) {
+  void complete([FutureOr<T> value]) {
     if (!future._mayComplete) throw new StateError("Future already completed");
     future._asyncComplete(value);
   }
 
   void _completeError(Object error, StackTrace stackTrace) {
     future._asyncCompleteError(error, stackTrace);
   }
 }
 
 class _SyncCompleter<T> extends _Completer<T> {
-  void complete([value]) {
+  void complete([FutureOr<T> value]) {
     if (!future._mayComplete) throw new StateError("Future already completed");
     future._complete(value);
   }
 
   void _completeError(Object error, StackTrace stackTrace) {
     future._completeError(error, stackTrace);
   }
 }
 
 class _FutureListener<S, T> {
@@ skipping 676 matching lines @@
       }
     }, onError: (e, s) {
       if (timer.isActive) {
         timer.cancel();
         result._completeError(e, s);
       }
     });
     return result;
   }
 }