Accept a computation that returns a Future for Future.delayed.

R=lrn@google.com

Review URL: https://codereview.chromium.org//1296623004 .

sdk/lib/async/future.dart

@@ -221,7 +221,7 @@ abstract class Future<T> {
    * See also [Completer] for a way to create and complete a future at a
    * later time that isn't necessarily after a known fixed duration.
    */
-  factory Future.delayed(Duration duration, [T computation()]) {
+  factory Future.delayed(Duration duration, [computation()]) {
     _Future result = new _Future<T>();
     new Timer(duration, () {
       try {

tests/lib/async/future_constructor2_test.dart

@@ -0,0 +1,62 @@
+// 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.
+
+library future_delayed_test;
+
+import 'package:async_helper/async_helper.dart';
+import "package:expect/expect.dart";
+import 'dart:async';
+
+Future<int> createIntFuture() {
+  return new Future<int>.value(499);
+}
+
+unnamed() {
+  asyncStart();
+  new Future<int>(createIntFuture)
+      .then((x) {
+    Expect.equals(499, x);
+    asyncEnd();
+  });
+}
+
+delayed() {
+  asyncStart();
+  new Future<int>.delayed(const Duration(milliseconds: 2), createIntFuture)
+      .then((x) {
+    Expect.equals(499, x);
+    asyncEnd();
+  });
+}
+
+microtask() {
+  asyncStart();
+  new Future<int>.microtask(createIntFuture)
+      .then((x) {
+    Expect.equals(499, x);
+    asyncEnd();
+  });
+}
+
+sync() {
+  asyncStart();
+  new Future<int>.sync(createIntFuture)
+      .then((x) {
+    Expect.equals(499, x);
+    asyncEnd();
+  });
+}
+
+main() {
+  asyncStart();
+  // Test that all the Future constructors take functions that return a Future
+  // as argument.
+  // In particular the constructors must not type their argument as
+  // `T computation()`.
+  unnamed();
+  delayed();
+  microtask();
+  sync();
+  asyncEnd();
+}