pkg/shelf: change helper method on Request and Response

pkg/shelf/test/message_change_test.dart

Index: pkg/shelf/test/message_change_test.dart
diff --git a/pkg/shelf/test/message_change_test.dart b/pkg/shelf/test/message_change_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..f77edd126fbca0650cf845a64fdc104120905431
--- /dev/null
+++ b/pkg/shelf/test/message_change_test.dart
@@ -0,0 +1,75 @@
+// Copyright (c) 2014, 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.
+
+library shelf.message_change_test;
+
+import 'package:unittest/unittest.dart';
+
+import 'package:shelf/shelf.dart';
+import 'package:shelf/src/message.dart';
+
+import 'test_util.dart';
+
+void main() {
+  group('Request', () {
+    _testChange(({headers, context}) {
+      return new Request('GET', LOCALHOST_URI, headers: headers,
+          context: context);
+    });
+  });
+
+  group('Response', () {
+    _testChange(({headers, context}) {
+      return new Response.ok(null, headers: headers,
+          context: context);
+    });
+  });
+}
+
+/// Shared test method used by [Request] and [Response] tests to validate
+/// the behavior of `change` with different `headers` and `context` values.
+void _testChange(Message factory({Map<String, String> headers,
+  Map<String, Object> context})) {
+  test('with empty headers returns indentical instance', () {
+    var request = factory(headers: {'header1': 'header value 1'});
+    var copy = request.change(headers: {});
+
+    expect(copy.headers, same(request.headers));
+  });
+
+  test('with empty context returns identical instance', () {
+    var request = factory(context: {'context1': 'context value 1'});
+    var copy = request.change(context: {});
+
+    expect(copy.context, same(request.context));
+  });
+
+  test('new header values are added', () {
+    var request = factory(headers: {'test':'test value'});
+    var copy = request.change(headers: {'test2': 'test2 value'});
+
+    expect(copy.headers, {'test':'test value', 'test2':'test2 value'});
+  });
+
+  test('existing header values are overwritten', () {
+    var request = factory(headers: {'test':'test value'});
+    var copy = request.change(headers: {'test': 'new test value'});
+
+    expect(copy.headers, {'test':'new test value'});
+  });
+
+  test('new context values are added', () {
+    var request = factory(context: {'test':'test value'});
+    var copy = request.change(context: {'test2': 'test2 value'});
+
+    expect(copy.context, {'test':'test value', 'test2':'test2 value'});
+  });
+
+  test('existing context values are overwritten', () {
+    var request = factory(context: {'test':'test value'});
+    var copy = request.change(context: {'test': 'new test value'});
+
+    expect(copy.context, {'test':'new test value'});
+  });
+}