prefix property
Which entries this covers: a path relative to the package, in the same
coordinates as PreviewsPackage.directory and the paths the tool reports
entries under.
The empty string covers the whole package, which is what a project with
one form factor writes and what a bare device: on the package means.
Matched on segment boundaries, never as raw text — src/mobile covers
src/mobile/tile.dart and pointedly not src/mobile_legacy/tile.dart.
A file is a legal prefix, and supported rather than incidental: the
last segment of a path is a file, so src/mobile/tile.dart covers exactly
the entries in that one file. It is how a single preview differs from its
neighbours without giving it a directory of its own. This said "a
directory" for a while, and a consumer who tried a file, watched it work
and could not tell whether they were allowed to depend on it is the reason
it does not any more.
Implementation
final String prefix;