allowFileSystemImports property
DGUB3 (mirrors tom_d4rt DFUB1) — when true, _fetchModuleSource may read
a module's source from the filesystem (via _resolveFileSystemUri) if the
URI is not already in sources and the resolved file exists. Defaults to
false (sandboxed: only preloaded sources and bridged/stdlib modules
are visible). Every such read is gated by
_checkFileSystemSourceReadPermission.
Implementation
final bool allowFileSystemImports;