[gardening] Fix update_error_cmdline_test to handle windows.

Follow-up to 409ecc7

TEST=ci

Change-Id: I803eb8d1352fe0eca14e89afefde2eb12b0f6854
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/450742
Reviewed-by: Bob Nystrom <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

pkg/test_runner/test/update_error_cmdline_test.dart

@@ -16,10 +16,10 @@ void main() async {
   const expectedLineCfe = 'Running CFE on 1 file...';
   final expectedUpdateTextCfe = '${testNamePartsCfe.last}.dart (3 errors)';
   final expectedLineCfeWeb =
-      'Running dart2js on ${toFileName(toFileNameParts(testNamePartsCfe))}...';
+      'Running dart2js on ${toFileNameParts(testNamePartsCfe).join('/')}...';
 
   final expectedLineWeb =
-      'Running dart2js on ${toFileName(toFileNameParts(testNamePartsWeb))}...';
+      'Running dart2js on ${toFileNameParts(testNamePartsWeb).join('/')}...';
   final expectedUpdateTextWeb = '${testNamePartsWeb.last}.dart (1 error)';
 
   await doTest(toFileNameParts(testNamePartsCfe), testNamePartsCfe,