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 3d25a2f8ec65a57491d2f18af6654ff8566cd414..45630c743a64fba5e97e5d46c0f464799d739941 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', () {