We want:

• Should be invokable in any directory. The relevant enclosing directory with a package config should be found.
• a fast-path for dart run that does not have to parse yaml.
• Only .dart_tool/package_config.json should be parsed, and relevant timestamps compared.
• If .dart_tool/package_config.json could not be found, or time-stamps mismatch we do a full resolution.
• A stray package config above the workspaceroot pubspec.yaml should not be used.

The procedure for the fast path would be like:

We then for each parent directory p of current directory (including it):

• If p is the file system root: use an empty package config.
• look for $p/.dart_tool/package_config.json. If that exists check if it is up-to-date. (this allows a stand-alone package config).
• look for $p/pubspec.yaml:
  • If that exists: look for $p/.dart_tool/pub/workspace_ref.json.
    • If that exists: check if the .dart_tool/package_config.json relative to its "workspaceRoot" is up-to-date.
    • Otherwise: do full resolution
  • Otherwise continue to next parent directory.