An SDK directory that exists but cannot be read passed discovery, and the
analyzer then died with a raw FileSystemException stack trace. This
happens for real under macOS privacy protection: a Flutter SDK under
~/Documents stats fine but every open fails, and every app whose
package_config.json names that flutterRoot crashed:
Unhandled exception:
FileSystemException(path=.../dart-sdk/lib/_internal/allowed_experiments.json; ...)
The chosen SDK is now probed before the analysis collection is built, and
an unreadable one is a PipeException naming the path, where it was
discovered from, and the --sdk remedy.
The last-resort fallback to the running Dart SDK took the executable's
directory (<sdk>/bin) as the SDK root, which never contains
lib/libraries.json, so it could never succeed. sdkFromExecutable now
maps <sdk>/bin/dart to <sdk>.
