Added a `ResultFuture` class.

test/result_future_test.dart

Index: test/result_future_test.dart
diff --git a/test/result_future_test.dart b/test/result_future_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..d8b4e01e2778357c0a36b1d1cdd42749a73dafa5
--- /dev/null
+++ b/test/result_future_test.dart
@@ -0,0 +1,44 @@
+// 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));
+    });
+  });
+}