Refactor Future constructors.

tests/lib/async/slow_consumer3_test.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.
 
 // VMOptions=--old_gen_heap_size=64
 
 library slow_consumer3_test;
 
 import "package:expect/expect.dart";
 import 'dart:async';
@@ skipping 40 matching lines @@
         }
       },
       onDone: () {
         finalCount = receivedCount;
         result.complete(receivedCount);
       });
     return result.future;
   }
 
   Future close() {
-    return new Future.immediate(finalCount);
+    return new Future.value(finalCount);
   }
 }
 
 Stream<List> dataGenerator(int bytesTotal, int chunkSize) {
   int chunks = bytesTotal ~/ chunkSize;
   return new Stream.fromIterable(new Iterable.generate(chunks, (_) {
     // This assumes one byte per entry. In practice it will be more.
     return new List<int>(chunkSize);
   }));
 }
@@ skipping 13 matching lines @@
   // file). If the consumer doesn't pause the data-provider it will run out of
   // heap-space.
 
   dataGenerator(100 * MB, 512 * KB)
     .pipe(new SlowConsumer(200 * MB, 3 * MB))
     .then((count) {
       port.close();
       Expect.equals(100 * MB, count);
     });
 }