scenarioAmbientFile top-level property

The file whose main() is declaring right now, or null outside the harness.

Armed beside scenarioDeclarationSink and for the same reason: under the runner one process declares every file, so nothing a scenario knows about itself is unique to it. A description is not — two files may each hold a scenario('Home'), and anything this package keys on the description alone silently treats them as one scenario. The standalone lane needs no such thing, because flutter test gives each file its own process.