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

lib/src/result/error.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 'value.dart';
 
 /// A result representing a thrown error.
-class ErrorResult<T> implements Result<T> {
+class ErrorResult implements Result<Null> {
   final error;
   final StackTrace stackTrace;
 
   bool get isValue => false;
   bool get isError => true;
-  ValueResult<T> get asValue => null;
-  ErrorResult<T> get asError => this;
+  ValueResult<Null> get asValue => null;
+  ErrorResult<Null> get asError => this;
 
   ErrorResult(this.error, this.stackTrace);
 
   void complete(Completer completer) {
     completer.completeError(error, stackTrace);
   }
 
   void addTo(EventSink sink) {
     sink.addError(error, stackTrace);
   }
 
-  Future<T> get asFuture => new Future.error(error, stackTrace);
+  Future<Null> get asFuture => new Future<Null>.error(error, stackTrace);
 
   /// Calls an error handler with the error and stacktrace.
   ///
   /// An async error handler function is either a function expecting two
   /// arguments, which will be called with the error and the stack trace, or it
   /// has to be a function expecting only one argument, which will be called
   /// with only the error.
   void handle(Function errorHandler) {
     if (errorHandler is ZoneBinaryCallback) {
       errorHandler(error, stackTrace);
     } else {
       errorHandler(error);
     }
   }
 }