Fix path.toUri for relative paths.

test/windows_test.dart

Index: test/windows_test.dart
diff --git a/test/windows_test.dart b/test/windows_test.dart
index a174305a745cdeb2f5fb23292a6647c8762cf4d3..69428378dc2a6496924f6f83f736e295661b5b27 100644
--- a/test/windows_test.dart
+++ b/test/windows_test.dart
@@ -629,6 +629,7 @@ main() {
         context.toUri(r'C:\path\to\foo'), Uri.parse('file:///C:/path/to/foo'));
     expect(context.toUri(r'C:\path\to\foo\'),
         Uri.parse('file:///C:/path/to/foo/'));
+    expect(context.toUri(r'path\to\foo\'), Uri.parse('path/to/foo/'));
     expect(context.toUri(r'C:\'), Uri.parse('file:///C:/'));
     expect(context.toUri(r'\\server\share'), Uri.parse('file://server/share'));
     expect(