Add reactive-inspired stream transformers:

lib/result.dart

Index: lib/result.dart
diff --git a/lib/result.dart b/lib/result.dart
index 13e3dd94b288d8e579f3cc8a77eb88430f179356..41172e6377af270fded18bf7423f4743b55746fa 100644
--- a/lib/result.dart
+++ b/lib/result.dart
@@ -129,6 +129,19 @@ abstract class Result<T> {
 
   /// 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.
@@ -146,6 +159,18 @@ class ValueResult<T> implements Result<T> {
     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.
@@ -178,6 +203,19 @@ class ErrorResult implements Result {
       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.