Remove Futures class, move methods to Future.

tests/standalone/io/directory_fuzz_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.
 
 // 'fuzz' test the directory APIs by providing unexpected type
 // arguments. The test passes if the VM does not crash.
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
@@ skipping 41 matching lines @@
     futures.add(doItAsync(() {
       return d.exists().then((res) {
         if (!res) return d.delete(recursive: true);
         return new Future.immediate(true);
       });
     }));
     typeMapping.forEach((k2, v2) {
       futures.add(doItAsync(() => d.rename(v2)));
     });
   });
-  Futures.wait(futures).then((ignore) => port.close());
+  Future.wait(futures).then((ignore) => port.close());
 }
 
 main() {
   fuzzSyncMethods();
   fuzzAsyncMethods();
 }