Copying over a symlink clobbers the symlink instead of its target on Windows

[jamesderlin]

Steps to reproduce:

1. On a Windows 10 machine with Dart 2.10, run the following code, which copies a file with a symlink as the destination.

import 'dart:io' as io;

void main() {
  var f = io.File('foo')..createSync()..writeAsStringSync('foo');
  io.File('bar')..createSync()..writeAsStringSync('bar');
  io.Link('symlink').createSync('bar');
  f.copySync('.\\symlink');
  print(io.FileSystemEntity.typeSync('symlink', followLinks: false));  // Expect: FileSystemEntityType:<link>
  print(io.File('bar').readAsStringSync()); // Expect: foo
}

Expected result:

Expected output is:

FileSystemEntityType:<link>
foo

which indicates that the copied file overwrites the symlink target instead of the symlink itself.

Actual result:

With Dart SDK version: 2.10.0-7.3.beta (beta) (Wed Aug 26 09:46:50 2020 +0200) on "windows_x64", I get:

FileSystemEntityType:<file>
bar

The copied file overwrote the symlink itself, not the symlink's target.

Additional details:

• This works as expected with Dart SDK version: 2.9.3 (stable) (Tue Sep 8 11:21:00 2020 +0200) on "windows_x64", so this is a regression.
• This seems to happen only if copySync's destination includes an explicit directory. That is, I get the expected behavior if I use f.copySync('symlink'); instead of f.copySync('.\\symlink').
• This seems Windows-specific. (I tested on Windows 10 version 2004.)

This has been causing CI failures for package:file's LocalFileSystem tests on Windows.

[tvolkert]

@a-siva (relatively recent regression)

[a-siva]

/cc @aam