Refactor Future constructors.

pkg/http/lib/src/utils.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.
 
 library utils;
 
 import 'dart:async';
 import 'dart:crypto';
 import 'dart:io';
 import 'dart:typeddata';
@@ skipping 146 matching lines @@
 /// done.
 Future writeStreamToSink(Stream stream, EventSink sink) {
   var completer = new Completer();
   stream.listen(sink.add,
       onError: sink.addError,
       onDone: () => completer.complete());
   return completer.future;
 }
 
 /// Returns a [Future] that asynchronously completes to `null`.
-Future get async => new Future.immediate(null);
+Future get async => new Future.value();
 
 /// Returns a closed [Stream] with no elements.
 Stream get emptyStream => streamFromIterable([]);
 
 /// Creates a single-subscription stream that emits the items in [iter] and then
 /// ends.
 Stream streamFromIterable(Iterable iter) {
   var controller = new StreamController();
   iter.forEach(controller.add);
   controller.close();
@@ skipping 49 matching lines @@
 // is in.
 /// Runs [fn] for each element in [input] in order, moving to the next element
 /// only when the [Future] returned by [fn] completes. Returns a [Future] that
 /// completes when all elements have been processed.
 ///
 /// The return values of all [Future]s are discarded. Any errors will cause the
 /// iteration to stop and will be piped through the return value.
 Future forEachFuture(Iterable input, Future fn(element)) {
   var iterator = input.iterator;
   Future nextElement(_) {
-    if (!iterator.moveNext()) return new Future.immediate(null);
+    if (!iterator.moveNext()) return new Future.value();
     return fn(iterator.current).then(nextElement);
   }
   return nextElement(null);
 }