Add extra Path test.

tests/standalone/io/path_test.dart

Index: tests/standalone/io/path_test.dart
diff --git a/tests/standalone/io/path_test.dart b/tests/standalone/io/path_test.dart
index b8b58b6f38489e1a934300a76c415f08d9b41bd5..5e972f5976654c3d41e8084ef6c9e690f3669eb9 100644
--- a/tests/standalone/io/path_test.dart
+++ b/tests/standalone/io/path_test.dart
@@ -323,6 +323,7 @@ void testWindowsDrive() {
   var C2Path = new Path(r'C:\a\b\d');
   var C3Path = new Path(r'C:\a\b');
   var C4Path = new Path(r'C:\');
+  var c4Path = new Path(r'c:\');
   var DPath = new Path(r'D:\a\b\d\e');
   var NoPath = new Path(r'\a\b\c\.');
 
@@ -338,4 +339,5 @@ void testWindowsDrive() {
   Expect.equals('d', cPath.relativeTo(C3Path).toString());
   Expect.equals('a/b/d', cPath.relativeTo(C4Path).toString());
   Expect.equals('../../../', C4Path.relativeTo(cPath).toString());
+  Expect.equals('a/b', C3Path.relativeTo(c4Path).toString());
 }