In our overrides WebSocket.send is void.

Source/WebCore/bindings/dart/custom/DartWebSocketCustom.cpp

 // Copyright 2011, Google Inc.
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //    * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //    * Redistributions in binary form must reproduce the above
@@ skipping 92 matching lines @@
             result = receiver->send(data, ec);
         } else {
             exception = Dart_NewString("Unsupported data argument type");
             goto fail;
         }
 
         if (UNLIKELY(ec)) {
             exception = DartDOMWrapper::exceptionCodeToDartException(ec);
             goto fail;
         }
-        Dart_SetReturnValue(args, DartUtilities::boolToDart(result));
         return;
     }
 
 fail:
     Dart_ThrowException(exception);
     ASSERT_NOT_REACHED();
 }
 
 }
 
 }
 
 #endif // ENABLE(WEB_SOCKETS)