Add a --log-file option to the MCP Server

[DanTup]

When using the "dart mcp-server" command (for example via Dart-Code registering it with VS Code) it would be useful to be able to capture communications to a file (which we support for other spawned processes in Dart-Code). Because VS Code starts the MCP server itself, we don't have access to the process to log the streams ourselves as we usually would.

[jakemac53]

What should the behavior be for this if the file already exists? Append or overwrite?

[DanTup]

I'd say replace, because it avoids growing forever.

Dart-Code does overwrite the log files. The analysis server (for the instrumentation-log, which is logged by the server because it contains non-protocol things like watch events) rolls the logs over with suffixes keeping the last 5.

[jakemac53]

Ok, I will just opt to replace, I don't want to overcomplicate the logic with the rolling logs.

There is some risk of losing useful info but 🤷‍♂ . We can always adapt later on. Or, clients can implement the rolling logs themselves.

[DanTup]

@jakemac53 am I right in thinking that once this rolls into the SDK, it will need changes to lib\src\commands\dart_mcp_server.dart to forward the argument on? (because if I roll it into the SDK locally, it seems to refuse to recognise this flag, even though running the AOT snapshot directly does handle it)

[jakemac53]

Yes, we will need to plumb the flag through to the SDK command, it has its own arg parser.

Also, I seem to have broken something with this flag so that it causes the server to stop working when it is provided, so I also need to look into that.

[jakemac53]

See #181