Throw exception when trying to modify HTTP status code when header is already sent

tests/standalone/io/http_headers_state_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 "package:expect/expect.dart";
 import "dart:isolate";
 import "dart:io";
 
 void test(int totalConnections, [String body]) {
   HttpServer.bind("127.0.0.1", 0).then((server) {
 
     server.listen((HttpRequest request) {
       HttpResponse response = request.response;
       // Cannot mutate request headers.
       Expect.throws(() => request.headers.add("X-Request-Header", "value"),
                     (e) => e is HttpException);
       Expect.equals("value", request.headers.value("X-Request-Header"));
       request.listen((_) {}, onDone: () {
         // Can still mutate response headers as long as no data has been sent.
         response.headers.add("X-Response-Header", "value");
         if (body != null) {
           response.write(body);
+          // Cannot change state or reason when data has been sent.
+          Expect.throws(() => response.statusCode = 200,
+                        (e) => e is StateError);
+          Expect.throws(() => response.reasonPhrase = "OK",
+                        (e) => e is StateError);
           // Cannot mutate response headers when data has been sent.
-          Expect.throws(() => request.headers.add("X-Request-Header", "value2"),
+          Expect.throws(() => response.headers.add("X-Request-Header", "value2"),
                         (e) => e is HttpException);
         }
         response..close();
-        // Cannot mutate response headers when data has been sent.
-        Expect.throws(() => request.headers.add("X-Request-Header", "value3"),
+        // Cannot change state or reason after connection is closed.
+        Expect.throws(() => response.statusCode = 200,
+                      (e) => e is StateError);
+        Expect.throws(() => response.reasonPhrase = "OK",
+                      (e) => e is StateError);
+        // Cannot mutate response headers after connection is closed.
+        Expect.throws(() => response.headers.add("X-Request-Header", "value3"),
                       (e) => e is HttpException);
       });
     });
 
     int count = 0;
     HttpClient client = new HttpClient();
     for (int i = 0; i < totalConnections; i++) {
       client.get("127.0.0.1", server.port, "/")
         .then((HttpClientRequest request) {
           if (body != null) {
@@ skipping 31 matching lines @@
         });
     }
 
   });
 }
 
 void main() {
   test(5);
   test(5, "Hello and goodbye");
 }