Allow additional headers for WebSocket connect

sdk/lib/io/websocket_impl.dart

Index: sdk/lib/io/websocket_impl.dart
diff --git a/sdk/lib/io/websocket_impl.dart b/sdk/lib/io/websocket_impl.dart
index 872cd321ad7b223deadefd840311925fe9f123b8..6683bdeeb048d19a25272d77c7995bdbb8e60c65 100644
--- a/sdk/lib/io/websocket_impl.dart
+++ b/sdk/lib/io/websocket_impl.dart
@@ -769,7 +769,8 @@ class _WebSocketImpl extends Stream with _ServiceObject implements WebSocket {
 
   static final HttpClient _httpClient = new HttpClient();
 
-  static Future<WebSocket> connect(String url, List<String> protocols) {
+  static Future<WebSocket> connect(
+      String url, List<String> protocols, Function onRequest) {
     Uri uri = Uri.parse(url);
     if (uri.scheme != "ws" && uri.scheme != "wss") {
       throw new WebSocketException("Unsupported URL scheme '${uri.scheme}'");
@@ -800,12 +801,18 @@ class _WebSocketImpl extends Stream with _ServiceObject implements WebSocket {
             ..add(HttpHeaders.CONNECTION, "Upgrade")
             ..set(HttpHeaders.UPGRADE, "websocket")
             ..set("Sec-WebSocket-Key", nonce)
-            ..set("Cache-Control", "no-cache")

[kustermann, 2015/01/09 14:45:12]

Why was this removed?

[Søren Gjesse, 2015/01/12 15:36:34]

I thought it was not needed as a proxy should never cache the response to an
upgrade request. However that is of cause not always true, so added it back.

             ..set("Sec-WebSocket-Version", "13");
         if (protocols.isNotEmpty) {
           request.headers.add("Sec-WebSocket-Protocol", protocols);
         }
-        return request.close();
+        if (onRequest != null) {
+          var result = onRequest(request);
+          return result is Future

[Lasse Reichstein Nielsen, 2015/01/12 15:11:56]

Parentheses around (result is Future) expression (and generally around any
non-trivial condition expression, where trivial really means "without spaces")

[Søren Gjesse, 2015/01/12 15:36:34]

Dropped this code.

+              ? result.then((_) => request.close())
+              : request.close();
+        } else {
+          return request.close();
+        }

[Lasse Reichstein Nielsen, 2015/01/12 15:11:56]

Rewrite as:

if (onRequest != null) {
  var result = onRequest(request);  
  if (result is Future) {
    return result.then((_) => request.close());
  }
}
return request.close();


Is the code safe from memory leaks or similar if onRequest throws?

[Søren Gjesse, 2015/01/12 15:36:34]

Dropped this code.

       })
       .then((response) {
         void error(String message) {