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

test/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';
-
-void main() {
-  var completer;
-  var future;
-  setUp(() {
-    completer = new Completer();
-    future = new ResultFuture(completer.future);
-  });
-
-  test('before completion, result is null', () {
-    expect(future.result, isNull);
-  });
-
-  test('after successful completion, result is the value of the future', () {
-    completer.complete(12);
-
-    // The completer calls its listeners asynchronously. We have to wait
-    // before we can access the result.
-    expect(future.then((_) => future.result.asValue.value),
-        completion(equals(12)));
-  });
-
-  test("after an error completion, result is the future's error", () {
-    var trace = new Trace.current();
-    completer.completeError('error', trace);
-
-    // 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));
-    });
-  });
-}