Fix type warning in Result.release.

lib/src/result.dart

 // Copyright (c) 2016, 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 'result/capture_transformer.dart';
 import 'result/error.dart';
 import 'result/release_transformer.dart';
 import 'result/value.dart';
@@ skipping 72 matching lines @@
   }
 
   /// Release the result of a captured future.
   ///
   /// Converts the [Result] value of the given [future] to a value or error
   /// completion of the returned future.
   ///
   /// If [future] completes with an error, the returned future completes with
   /// the same error.
   static Future<T> release<T>(Future<Result<T>> future) =>
-      future.then<Future<T>>((result) => result.asFuture);
+      future.then<T>((result) => result.asFuture);
 
   /// Capture the results of a stream into a stream of [Result] values.
   ///
   /// The returned stream will not have any error events.
   /// Errors from the source stream have been converted to [ErrorResult]s.
   static Stream<Result<T>> captureStream<T>(Stream<T> source) =>
       source.transform(new CaptureStreamTransformer<T>());
 
   /// Release a stream of [result] values into a stream of the results.
   ///
@@ skipping 39 matching lines @@
   void complete(Completer<T> completer);
 
   /// Add this result to an [EventSink].
   ///
   /// Calls the sink's `add` or `addError` method as appropriate.
   void addTo(EventSink<T> sink);
 
   /// Creates a future completed with this result as a value or an error.
   Future<T> get asFuture;
 }