Add methods to Result. Bump version to 2.0 and remove deprecated features.

test/result/result_future_test.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 'package:async/async.dart';
 import 'package:stack_trace/stack_trace.dart';
 import 'package:test/test.dart';
 
@@ skipping 24 matching lines @@
 
     // The completer calls its listeners asynchronously. We have to wait
     // before we can access the result.
     return future.catchError((_) {}).then((_) {
       var error = future.result.asError;
       expect(error.error, equals('error'));
       expect(error.stackTrace, equals(trace));
     });
   });
 }