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

sdk/lib/io/http_impl.dart

Index: sdk/lib/io/http_impl.dart
diff --git a/sdk/lib/io/http_impl.dart b/sdk/lib/io/http_impl.dart
index 42b8d6f80b16130378023831ef41427c87eaa0f6..fcd9a0621d831186dc1cbb77d51b2dc3c6850d73 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -697,7 +697,7 @@ class _BufferTransformer implements StreamTransformer<List<int>, List<int>> {
 
 class _HttpResponse extends _HttpOutboundMessage<HttpResponse>
     implements HttpResponse {
-  int statusCode = 200;
+  int _statusCode = 200;
   String _reasonPhrase;
   List<Cookie> _cookies;
   _HttpRequest _httpRequest;
@@ -717,6 +717,12 @@ class _HttpResponse extends _HttpOutboundMessage<HttpResponse>
     return _cookies;
   }
 
+  int get statusCode => _statusCode;
+  void set statusCode(int statusCode) {
+    if (_headersWritten) throw new StateError("Header already sent");
+    _statusCode = statusCode;
+  }
+
   String get reasonPhrase => _findReasonPhrase(statusCode);
   void set reasonPhrase(String reasonPhrase) {
     if (_headersWritten) throw new StateError("Header already sent");