Fix subdirectory output (#47256)

packages/flutter_test/lib/src/_goldens_io.dart

@@ -155,7 +155,7 @@ class LocalComparisonOutput {
 
   /// Returns the appropriate file for a given diff from a [ComparisonResult].
   File getFailureFile(String failure, Uri golden, Uri basedir) {
-    final String fileName = golden.pathSegments[0];
+    final String fileName = golden.pathSegments.last;
     final String testName = fileName.split(path.extension(fileName))[0]
       + '_'
       + failure

packages/flutter_test/test/goldens_test.dart

@@ -149,7 +149,7 @@ void main() {
 
       group('fails', () {
 
-        test('and generates correct output in the correct location', () async {
+        test('and generates correct output in the correct base location', () async {
           comparator = LocalFileComparator(Uri.parse('local_test.dart'), pathStyle: fs.path.style);
           await fs.file(fix('/golden.png')).writeAsBytes(_kColorFailurePngBytes);
           try {
@@ -176,6 +176,35 @@ void main() {
           }
         });
 
+        test('and generates correct output when files are in a subdirectory', () async {
+          comparator = LocalFileComparator(Uri.parse('local_test.dart'), pathStyle: fs.path.style);
+          fs.file(fix('subdir/golden.png'))
+            ..createSync(recursive:true)
+            ..writeAsBytes(_kColorFailurePngBytes);
+          try {
+            await doComparison('subdir/golden.png');
+            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();