This is a package of classes that are generated from the DAP specifications along with their generating code.
tool/external_dap_spec/debugAdapterProtocol.json is an unmodified copy of the DAP Specification, downloaded from:
This accompanying file is the version of the specification that was used to generate a portion of the Dart code used to support the protocol.
To regenerate the generated code, run the script in "tool/dap/generate_all.dart" with no arguments. To download the latest version of the specification before regenerating the code, run the same script with the "--download" argument.
More information on Dart support for DAP is available here.