Use FutureOr more and make Future.sync return the resulting Future directly.

tests/lib/async/future_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.
 
 library future_test;
 
 import 'package:async_helper/async_helper.dart';
 import "package:expect/expect.dart";
 import 'dart:async';
 
@@ skipping 976 matching lines @@
       testType(name, future.whenComplete(() {}), depth - 1);
     }
   }
 
   for (var value in [42, null]) {
     testType("Future($value)", new Future<int>(() => value));
     testType("Future.delayed($value)",
         new Future<int>.delayed(Duration.ZERO, () => value));
     testType(
         "Future.microtask($value)", new Future<int>.microtask(() => value));
-    testType("Future.sync($value)", new Future<int>.sync(() => value)); // //# 01: ok
-    testType("Future.sync(future($value))", //                             //# 01: continued
-             new Future<int>.sync(() async => new Future.value(value))); //# 01: continued
+    testType("Future.sync($value)", new Future<int>.sync(() => value)); //    //# 01: ok
+    testType("Future.sync(future($value))", //                                //# 01: continued
+             new Future<int>.sync(() async => new Future<int>.value(value))); //# 01: continued
     testType("Future.value($value)", new Future<int>.value(value));
   }
   testType("Completer.future", new Completer<int>().future);
   testType("Future.error", new Future<int>.error("ERR")..catchError((_) {}));
 }
 
 void testAnyValue() {
   asyncStart();
   var cs = new List.generate(3, (_) => new Completer());
   var result = Future.any(cs.map((x) => x.future));
@@ skipping 221 matching lines @@
     return (new StreamController()
           ..add(_result)
           ..close())
         .stream;
   }
 
   Future timeout(Duration duration, {onTimeout()}) {
     return this;
   }
 }