Currently, this is not yet possible in Dart. If you change the code, you must restart the application.
I would not expect this to work in the near future. The main feature that allows you to do this is the manipulation of the code at runtime. This is planned, but, as far as I know, has not yet begun.
EDIT
The above function is necessary if you want the code to be updated without losing the current state of the application, but usually this is not so important on the server, because in any case it should be (mostly) non-volatile.
In Dart, there is no need to restart the server application when only part of the client changes.
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