Fix most strong mode warnings.

lib/src/result/value.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.dart';
 import 'error.dart';
 
 /// A result representing a returned value.
 class ValueResult<T> implements Result<T> {
   final T value;
 
   bool get isValue => true;
   bool get isError => false;
   ValueResult<T> get asValue => this;
-  ErrorResult get asError => null;
+  ErrorResult<T> get asError => null;
 
   ValueResult(this.value);
 
   void complete(Completer<T> completer) {
     completer.complete(value);
   }
 
   void addTo(EventSink<T> sink) {
     sink.add(value);
   }
 
   Future<T> get asFuture => new Future.value(value);
 }