Fix most strong mode warnings.

lib/src/result/future.dart

 // Copyright (c) 2015, 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 'dart:async';
 
 import '../delegate/future.dart';
 import '../result.dart';
 
 /// A [Future] wrapper that provides synchronous access to the result of the
 /// wrapped [Future] once it's completed.
 class ResultFuture<T> extends DelegatingFuture<T> {
   /// Whether the future has fired and [result] is available.
   bool get isComplete => result != null;
 
   /// The result of the wrapped [Future], if it's completed.
   ///
   /// If it hasn't completed yet, this will be `null`.
   Result<T> get result => _result;
   Result<T> _result;
 
   factory ResultFuture(Future<T> future) {
-    var resultFuture;
-    resultFuture = new ResultFuture._(Result.capture(future).then((result) {
+    ResultFuture<T> resultFuture;
+    resultFuture = new ResultFuture._(() async {
+      var result = await Result.capture(future);
       resultFuture._result = result;
-      return result.asFuture;
-    }));
+      return await result.asFuture;
+    }());
     return resultFuture;
   }
 
   ResultFuture._(Future<T> future)
       : super(future);
 }