Refactor Future constructors.

tests/lib/async/futures_test.dart

 // Copyright (c) 2011, 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 futures_test;
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:isolate';
 
 Future testWaitEmpty() {
@@ skipping 66 matching lines @@
 Future testForEachEmpty() {
   return Future.forEach([], (_) {
     throw 'should not be called';
   });
 }
 
 Future testForEach() {
   var seen = <int>[];
   return Future.forEach([1, 2, 3, 4, 5], (n) {
     seen.add(n);
-    return new Future.immediate(null);
+    return new Future.value();
   }).then((_) => Expect.listEquals([1, 2, 3, 4, 5], seen));
 }
 
 Future testForEachWithException() {
   var seen = <int>[];
   return Future.forEach([1, 2, 3, 4, 5], (n) {
     if (n == 4) throw 'correct exception';
     seen.add(n);
-    return new Future.immediate(null);
+    return new Future.value();
   }).then((_) {
     throw 'incorrect exception';
   }).catchError((e) {
     Expect.equals('correct exception', e.error);
   });
 }
 
 main() {
   List<Future> futures = new List<Future>();
 
   futures.add(testWaitEmpty());
   futures.add(testCompleteAfterWait());
   futures.add(testCompleteBeforeWait());
   futures.add(testWaitWithMultipleValues());
   futures.add(testWaitWithSingleError());
   futures.add(testWaitWithMultipleErrors());
   futures.add(testForEachEmpty());
   futures.add(testForEach());
   futures.add(testForEachWithException());
 
   // Use a receive port for blocking the test.
   // Note that if the test fails, the program will not end.
   ReceivePort port = new ReceivePort();
   Future.wait(futures).then((List list) {
     Expect.equals(9, list.length);
     port.close();
   });
 }