Handle relative paths where the trailing directory has an extension.

utils/tests/pub/path/path_posix_test.dart

Index: utils/tests/pub/path/path_posix_test.dart
diff --git a/utils/tests/pub/path/path_posix_test.dart b/utils/tests/pub/path/path_posix_test.dart
index f975194772b335a8a72494c9db31dc799b89cdf4..a34c32d57c2c2802e481b4a0c88cef8b52563a84 100644
--- a/utils/tests/pub/path/path_posix_test.dart
+++ b/utils/tests/pub/path/path_posix_test.dart
@@ -233,6 +233,11 @@ main() {
         expect(r.relative('a/./b/../c.txt'), 'a/c.txt');
       });
     });
+
+    test('from a root with extension', () {
+      var r = new path.Builder(style: path.Style.posix, root: '/dir.ext');
+      expect(r.relative('/dir.ext/file'), 'file');
+    });
   });
 
   group('resolve', () {