Add Result.flatten.

pkg/async/test/result_test.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.
 
 import "dart:async";
 import "dart:collection";
 import "package:async/result.dart";
 import "package:unittest/unittest.dart";
 
 void main() {
@@ skipping 213 matching lines @@
                     expect(e, equals("BAD"));
                     expect(s, same(stack));
                   }),
                   onDone: () { fail("Unexpected done event"); },
                   cancelOnError: true);
     c.add(new Result.value(42));
     c.add(new Result.error("BAD", stack));
     c.add(new Result.value(37));
     c.close();
   });
+
+
+  test("flatten error 1", () {
+    Result<int> error = new Result<int>.error("BAD", stack);
+    Result<int> flattened = Result.flatten(error);
+    expectResult(flattened, error);
+  });
+
+  test("flatten error 2", () {
+    Result<int> error = new Result<int>.error("BAD", stack);
+    Result<Result<int>> result = new Result<Result<int>>.value(error);
+    Result<int> flattened = Result.flatten(result);
+    expectResult(flattened, error);
+  });
+
+  test("flatten value", () {
+    Result<Result<int>> result =
+        new Result<Result<int>>.value(new Result<int>.value(42));
+    expectResult(Result.flatten(result), new Result<int>.value(42));
+  });
 }
 
 void expectResult(Result actual, Result expected) {
   expect(actual.isValue, equals(expected.isValue));
   expect(actual.isError, equals(expected.isError));
   if (actual.isValue) {
     expect(actual.asValue.value, equals(expected.asValue.value));
   } else {
     expect(actual.asError.error, equals(expected.asError.error));
     expect(actual.asError.stackTrace, same(expected.asError.stackTrace));
@@ skipping 12 matching lines @@
   void add(T value) { onData(value); }
   void addError(error, [StackTrace stack]) { onError(error, stack); }
   void close() { onDone(); }
 
   static void _nullData(value) { Expect.fail("Unexpected sink add: $value"); }
   static void _nullError(e, StackTrace s) {
     Expect.fail("Unexpected sink addError: $e");
   }
   static void _nullDone() { Expect.fail("Unepxected sink close"); }
 }