[analyzer] Fix windows test expectation in assist_processor.dart

Change-Id: Ic4665af3e28f5ce467c75c8a772c872f0ddf05df
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/436600
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

pkg/analysis_server/test/src/services/correction/assist/assist_processor.dart

@@ -86,7 +86,7 @@ abstract class AssistProcessorTest extends AbstractSingleUnitTest {
     var fileEdits = _change.edits;
     if (additionallyChangedFiles == null) {
       expect(fileEdits, hasLength(1));
-      expect(_change.edits[0].file, testFilePath);
+      expect(_change.edits[0].file, testFile.path);
       _resultCode = SourceEdit.applySequence(testCode, _change.edits[0].edits);
       expect(_resultCode, expected);
     } else {