* [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
* [flutter_tools] [dap] Ensure DAP sends app.stop/app.detach during terminate
Fixes an issue where the flutter_tester device may not be cleaned up correctly if we just terminate the Flutter process.
* Update integration test expectations
* Revert accidental commit
* [flutter_tools] [dap] Add support for passing env variables to spawned processes
* Use named args
* Use in-memory fs and FakePlatform
* Pass filesystem style to MemoryFileSystem
* Don't terminate Dart process pids from VM Service
These processes may be on another device, and in the case of attach the debugee should not be terminated anyway.
