Add a test to verify that virtual files can be written to (#251)

Author: brianquinlan

pkgs/io_file/lib/src/file_system.dart

@@ -137,8 +137,8 @@ class WriteMode {
 /// On Windows, paths refering to objects in the
 /// [win32 device namespace](https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file#win32-device-namespaces),
 /// such as named pipes, physical disks, and serial comnmunications ports
-/// (e.g. 'COM1'), must be prefixed with `r'\\.\'`. For example, `'COM1'` would
-/// be refered to by the path `r'\\.\COM1'`.
+/// (e.g. 'COM1'), must be prefixed with `r'\\.\'`. For example, `'NUL'` would
+/// be refered to by the path `r'\\.\NUL'`.
 @sealed
 abstract class FileSystem {
   /// Create a directory at the given path.

pkgs/io_file/lib/src/vm_windows_file_system.dart

@@ -331,8 +331,8 @@ final class WindowsMetadata implements Metadata {
 /// On Windows, paths refering to objects in the
 /// [win32 device namespace](https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file#win32-device-namespaces),
 /// such as named pipes, physical disks, and serial comnmunications ports
-/// (e.g. 'COM1'), must be prefixed with `r'\\.\'`. For example, `'COM1'` would
-/// be refered to by the path `r'\\.\COM1'`.
+/// (e.g. 'COM1'), must be prefixed with `r'\\.\'`. For example, `'NUL'` would
+/// be refered to by the path `r'\\.\NUL'`.
 final class WindowsFileSystem extends FileSystem {
   @override
   bool same(String path1, String path2) => using((arena) {

pkgs/io_file/test/write_as_bytes_test.dart

@@ -180,6 +180,16 @@ void main() {
         expect(File(path).readAsBytesSync(), [1, 2, 3] + data);
       });
 
+      test('null file', () {
+        final data = randomUint8List(20);
+
+        fileSystem.writeAsBytes(
+          Platform.isWindows ? r'\\.\NUL' : '/dev/null',
+          Uint8List.fromList(data),
+          WriteMode.appendExisting,
+        );
+      });
+
       test('failExisting', () {
         final data = randomUint8List(20);
         final path = '$tmp/file';