Add reactive-inspired stream transformers:

lib/result.dart

 // Copyright (c) 2013, 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.
 
 /// Capture asynchronous results into synchronous values.
 ///
 /// Capturing a result (either a returned value or a thrown error)
 /// means converting it into a [Result] -
 /// either a [ValueResult] or an [ErrorResult].
 ///
@@ skipping 111 matching lines @@
   /// Complete a completer with this result.
   void complete(Completer<T> completer);
 
   /// Add this result to an [EventSink].
   ///
   /// Calls the sink's `add` or `addError` method as appropriate.
   void addTo(EventSink<T> sink);
 
   /// Creates a future completed with this result as a value or an error.
   Future<T> get asFuture;
+
+  /// This result is equal to another result with the same value or error.
+  ///
+  /// If this result is a value result, it is equal to another value result
+  /// with an equal value.
+  ///
+  /// If this result is an error result, it is equal to another error result
+  /// with equal error object and stack trace.
+  /// Notice that error objects and stack traces rarely override equality.
+  bool operator==(Object other);
+
+  /// A hash code compatible with [operator==].
+  int get hashCode;
 }
 
 /// A result representing a returned value.
 class ValueResult<T> implements Result<T> {
   final T value;
   ValueResult(this.value);
   bool get isValue => true;
   bool get isError => false;
   ValueResult<T> get asValue => this;
   ErrorResult get asError => null;
   void complete(Completer<T> completer) {
     completer.complete(value);
   }
   void addTo(EventSink<T> sink) {
     sink.add(value);
   }
   Future<T> get asFuture => new Future.value(value);
+
+  bool operator==(Object other) {
+    if (identical(this, other)) return true;
+    if (other is ValueResult) {
+      return value == other.value;
+    }
+    return false;
+  }
+
+  int get hashCode => value.hashCode;
+
+  String toString() => "Value($value)";
 }
 
 /// A result representing a thrown error.
 class ErrorResult implements Result {
   final error;
   final StackTrace stackTrace;
   ErrorResult(this.error, this.stackTrace);
   bool get isValue => false;
   bool get isError => true;
   ValueResult get asValue => null;
@@ skipping 12 matching lines @@
   /// 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);
     }
   }
+
+  bool operator==(Object other) {
+    if (identical(this, other)) return true;
+    if (other is ErrorResult) {
+      return error == other.error && stackTrace == other.stackTrace;
+    }
+    return false;
+  }
+
+  int get hashCode =>
+      ((error.hashCode * 31) + stackTrace.hashCode) & 0x3FFFFFFF;
+
+  String toString() => "Error($error${stackTrace == null ? "" : ", ..."})";
 }
 
 /// A stream transformer that captures a stream of events into [Result]s.
 ///
 /// The result of the transformation is a stream of [Result] values and
 /// no error events.
 class CaptureStreamTransformer<T> implements StreamTransformer<T, Result<T>> {
   const CaptureStreamTransformer();
 
   Stream<Result<T>> bind(Stream<T> source) {
@@ skipping 56 matching lines @@
     }
   }
   void addError(Object error, [StackTrace stackTrace]) {
     // Errors may be added by intermediate processing, even if it is never
     // added by CaptureSink.
     _sink.addError(error, stackTrace);
   }
 
   void close() { _sink.close(); }
 }