Fix websocket_impl.dart to work with websockets created by Chrome devtools.

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 6e2137f0543157646389b7c87cf2ea8edc1c7456..b169afac76c40f4d8f8a88b695b861e6599c162f 100644
--- a/sdk/lib/io/websocket_impl.dart
+++ b/sdk/lib/io/websocket_impl.dart
@@ -812,9 +812,10 @@ class _WebSocketImpl extends Stream implements WebSocket {
       .then((request) {
         // Setup the initial handshake.
         request.headers
-            ..add(HttpHeaders.CONNECTION, "upgrade")
+            ..add(HttpHeaders.CONNECTION, "Upgrade")
             ..set(HttpHeaders.UPGRADE, "websocket")
             ..set("Sec-WebSocket-Key", nonce)
+            ..set("Cache-Control", "no-cache")
             ..set("Sec-WebSocket-Version", "13");
         if (protocols.isNotEmpty) {
           request.headers.add("Sec-WebSocket-Protocol", protocols);