This is more than a cosmetic issue.
sun.nio.fs.DefaultFileSystemProvider#instance() leaks the platform-dependent type to platform-independent code.

Currently, callers of sun.nio.fs.DefaultFileSystemProvider#instance() must issue calls with different signatures on every OS because the target method signature needlessly use {Linux|Windows|Aix|MacOSX}FileSystemProvider, instead of the common FileSystemProvider type.

For example, java.nio.file.FileSystems.DefaultFileSystemHolder#getDefaultProvider() generates different bytecodes (INVOKESTATIC with different signatures) on every OS.

We stumbled upon this leakage while working on Espresso (a spec-compliant JVM written in Java, part of the GraalVM project), where we provide an alternative file system to overcome the deprecation of the SecurityManager.

While we have a workaround in Espresso, a tiny cleanup upstream is desirable.

These are internal APIs, there's no risk of breakage of any user code.