fix windows failures (#2244)

Fixes #2243 by not creating files that end in a `.`.

https://gist.github.com/jakemac53/845b60847dbda53f93aeed8056d5fbd2 illustrates what I think is the core of the issue. I would expect this to print false and also hit the catch block for every case. However, on 3.4.0, in the very first case the copy actually succeeds even though it claims the file doesn't exist. I filed dart-lang/sdk#55972 about this.

pkgs/test_core/CHANGELOG.md

@@ -1,6 +1,7 @@
 ## 0.6.5-wip
 
 * Increase SDK constraint to ^3.4.0.
+* Ensure we don't create files ending in a `.`, this breaks windows.
 
 ## 0.6.4
 

pkgs/test_core/lib/src/runner/package_version.dart

@@ -18,6 +18,6 @@ Future<String> get rootPackageLanguageVersionComment =>
 final _rootPackageLanguageVersionComment = DetachingFuture(() async {
   var packageConfig = await loadPackageConfigUri(await packageConfigUri);
   var rootPackage = packageConfig.packageOf(Uri.file(p.absolute('foo.dart')));
-  if (rootPackage == null) return '';
+  if (rootPackage == null) return '// <unknown-language-version>';
   return '// @dart=${rootPackage.languageVersion}';
 }());