* [flutter_tools/dap] Add support for forwarding `flutter run --machine` requests to the DAP client
Currently the only request that Flutter sends to the client is `app.exposeUrl` though most of this code is generic to support other requests that may be added in future.
* Improve comment
* Fix thrown strings
* StateError -> DebugAdapterException
* Add a non-null assertion and assert
* Use DebugAdapterException to handle restartRequests sent before process starts
* Fix typo + use local var
* Don't try to actually send Flutter messages in tests because there's no process
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