Add optimized _OneByteString.toLowerCase.

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 ee56378d788a279caf39372d7084f05931c712b9..76ee77d7b97360c266a134170a851d1b3f733655 100644
--- a/sdk/lib/io/websocket_impl.dart
+++ b/sdk/lib/io/websocket_impl.dart
@@ -464,11 +464,11 @@ class _WebSocketTransformerImpl implements WebSocketTransformer {
     }
     bool isUpgrade = false;
     request.headers[HttpHeaders.CONNECTION].forEach((String value) {
-      if (value.toLowerCase() == "upgrade") isUpgrade = true;
+      if (_ASCII.toLowerCase(value) == "upgrade") isUpgrade = true;
     });
     if (!isUpgrade) return false;
     String upgrade = request.headers.value(HttpHeaders.UPGRADE);
-    if (upgrade == null || upgrade.toLowerCase() != "websocket") {
+    if (upgrade == null || _ASCII.toLowerCase(upgrade) != "websocket") {
       return false;
     }
     String version = request.headers.value("Sec-WebSocket-Version");
@@ -834,8 +834,8 @@ class _WebSocketImpl extends Stream implements WebSocket {
         if (response.statusCode != HttpStatus.SWITCHING_PROTOCOLS ||
             response.headers[HttpHeaders.CONNECTION] == null ||
             !response.headers[HttpHeaders.CONNECTION].any(
-                (value) => value.toLowerCase() == "upgrade") ||
-            response.headers.value(HttpHeaders.UPGRADE).toLowerCase() !=
+                (value) => _ASCII.toLowerCase(value) == "upgrade") ||
+            _ASCII.toLowerCase(response.headers.value(HttpHeaders.UPGRADE)) !=
                 "websocket") {
           error("Connection to '$uri' was not upgraded to websocket");
         }