Fix for broken LocalFileComparator output (#40399)

* Fixing LocalFileComparator output

* Fixing path separators for windows failures

* Windows path issues

* Testing

* ++

* ++

* Fixing base directory - windows

* Clean up

packages/flutter_test/lib/src/_goldens_io.dart

@@ -129,7 +129,10 @@ class LocalFileComparator extends GoldenFileComparator {
       + '_'
       + failure
       + '.png';
-    return File(_path.join('failures', testName));
+    return File(_path.join(
+      _path.fromUri(basedir),
+      _path.fromUri(Uri.parse('failures/$testName')),
+    ));
   }
 }
 

packages/flutter_test/test/goldens_test.dart

@@ -148,6 +148,34 @@ void main() {
       });
 
       group('fails', () {
+
+        test('and generates correct output in the correct location', () async {
+          comparator = LocalFileComparator(Uri.parse('local_test.dart'), pathStyle: fs.path.style);
+          await fs.file(fix('/golden.png')).writeAsBytes(_kColorFailurePngBytes);
+          try {
+            await doComparison();
+            fail('TestFailure expected but not thrown.');
+          } on TestFailure catch (error) {
+            expect(error.message, contains('% diff detected'));
+            final io.File master = fs.file(
+              fix('/failures/golden_masterImage.png')
+            );
+            final io.File test = fs.file(
+              fix('/failures/golden_testImage.png')
+            );
+            final io.File isolated = fs.file(
+              fix('/failures/golden_isolatedDiff.png')
+            );
+            final io.File masked = fs.file(
+              fix('/failures/golden_maskedDiff.png')
+            );
+            expect(master.existsSync(), isTrue);
+            expect(test.existsSync(), isTrue);
+            expect(isolated.existsSync(), isTrue);
+            expect(masked.existsSync(), isTrue);
+          }
+        });
+
         test('when golden file does not exist', () async {
           try {
             await doComparison();