Modernize the package's style.

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 implements Result {
+  final error;
+  final StackTrace stackTrace;
+
+  bool get isValue => false;
+  bool get isError => true;
+  ValueResult get asValue => null;
+  ErrorResult 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 get asFuture => new Future.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);
+    }
+  }
+}