fix unsound cast failures in tests

tests/language_strong/async_test.dart

 // Copyright (c) 2014, 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.
 
 import 'package:expect/expect.dart';
 
 import 'dart:async';
 
 topLevelFunction() async {}
 
@@ skipping 27 matching lines @@
 }
 
 class B {
   final _y;
   const B._internal(this._y);
 
   B() : _y = null;
 }
 
 main() {
-  var asyncReturn;
+  Future asyncReturn;
 
   asyncReturn = topLevelFunction();
   Expect.isTrue(asyncReturn is Future);
 
   asyncReturn = topLevelWithParameter(4);
   Expect.isTrue(asyncReturn is Future);
   asyncReturn.then((int result) => Expect.equals(result, 11));
 
   asyncReturn = topLevelGetter;
   Expect.isTrue(asyncReturn is Future);
@@ skipping 28 matching lines @@
   var moreNesting = (int shadowP1, String p2, num p3) {
     var z = 3;
     aa(int shadowP1) async {
       return foo + z + p3 + shadowP1;
     }
 
     return aa(6);
   };
   asyncReturn = moreNesting(1, "ignore", 2);
   Expect.isTrue(asyncReturn is Future);
-  asyncReturn.then((int result) => Expect.equals(result, 28));
+  asyncReturn.then((num result) => Expect.equals(result, 28));
 
   var checkAsync = (var someFunc) {
     var toTest = someFunc();
     Expect.isTrue(toTest is Future);
     toTest.then((int result) => Expect.equals(result, 4));
   };
   checkAsync(() async => 4);
 }