[path_provider_windows] Resolve FFI stabilization changes (flutter#3485)

Ensures that path_provider_windows works on current null safety builds.

Author: Tim Sneath

packages/path_provider/path_provider_windows/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.1.0-nullsafety.1
+
+* Bump win32 dependency to latest version.
+
 ## 0.1.0-nullsafety
 
 * Migrate to null safety

packages/path_provider/path_provider_windows/lib/src/path_provider_windows_real.dart

@@ -116,7 +116,7 @@ class PathProviderWindows extends PathProviderPlatform {
   /// [WindowsKnownFolder].
   Future<String> getPath(String folderID) {
     final pathPtrPtr = allocate<Pointer<Utf16>>();
-    final Pointer<GUID> knownFolderID = calloc<GUID>()..setGUID(folderID);
+    final Pointer<GUID> knownFolderID = calloc<GUID>()..ref.setGUID(folderID);
 
     try {
       final hr = SHGetKnownFolderPath(

packages/path_provider/path_provider_windows/pubspec.yaml

@@ -1,7 +1,7 @@
 name: path_provider_windows
 description: Windows implementation of the path_provider plugin
 homepage: https://github.com/flutter/plugins/tree/master/packages/path_provider/path_provider_windows
-version: 0.1.0-nullsafety
+version: 0.1.0-nullsafety.1
 
 flutter:
   plugin:
@@ -17,7 +17,7 @@ dependencies:
   flutter:
     sdk: flutter
   ffi: ^0.2.0-nullsafety.1
-  win32: ^2.0.0-nullsafety.8
+  win32: ^2.0.0-nullsafety.9
 
 dev_dependencies:
   flutter_test: