Make a Future returned by the action of Future.whenCompelete delay.

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;
 
 deprecatedFutureValue(_FutureImpl future) =>
   future._isComplete ? future._resultOrListeners : null;
 
 
@@ skipping 139 matching lines @@
     if (_hasValue) {
       return new _FutureWrapper(this);
     }
     if (!_isComplete) {
       return new _CatchErrorFuture(f, test).._subscribeTo(this);
     } else {
       return _handleError(f, test);
     }
   }
 
-  Future<T> whenComplete(void action()) {
+  Future<T> whenComplete(action()) {
     _WhenFuture<T> whenFuture = new _WhenFuture<T>(action);
     if (!_isComplete) {
       _addListener(whenFuture);
     } else if (_hasValue) {
       new Timer(0, (_) {
         T value = _resultOrListeners;
         whenFuture._sendValue(value);
       });
     } else {
       assert(_hasError);
@@ skipping 238 matching lines @@
 }
 
 /** Future returned by [Future.whenComplete]. */
 class _WhenFuture<T> extends _TransformFuture<T, T> {
   final _FutureAction _action;
 
   _WhenFuture(this._action);
 
   void _sendValue(T value) {
     try {
-      _action();
+      var result = _action();
+      if (result is Future) {
+        Future resultFuture = result;
+        result.then((_) {
+          _setValue(value);
+        }, onError: (AsyncError e) {
+          _setError(e);
+        });
+        return;
+      }
     } catch (e, s) {
       _setError(new AsyncError(e, s));
       return;
     }
     _setValue(value);
   }
 
   void _sendError(AsyncError error) {
     try {
-      _action();
+      var result = _action();
+      if (result is Future) {
+        Future resultFuture = result;
+        result.then((_) {
+          _setError(error);
+        }, onError: (AsyncError e) {
+          // TODO(lrn): Find a way to combine error into the
+          // resulting error.
+          _setError(e);
+        });
+        return;
+      }
     } catch (e, s) {
       error = new AsyncError.withCause(e, s, error);
     }
     _setError(error);
   }
 }
 
 /**
  * Thin wrapper around a [Future].
  *
  * This is used to return a "new" [Future] that effectively work just
  * as an existing [Future], without making this discoverable by comparing
  * identities.
  */
 class _FutureWrapper<T> implements Future<T> {
   final Future<T> _future;
 
   _FutureWrapper(this._future);
 
   Future then(function(T value), { onError(AsyncError error) }) {
     return _future.then(function, onError: onError);
   }
 
   Future catchError(function(AsyncError error), {bool test(var error)}) {
     return _future.catchError(function, test: test);
   }
 
-  Future whenComplete(void action()) {
+  Future whenComplete(action()) {
     return _future.whenComplete(action);
   }
 
   Stream<T> asStream() => new Stream.fromFuture(this);
 }