Fixing local golden output flake (#44307)

packages/flutter_goldens/lib/flutter_goldens.dart

@@ -506,10 +506,11 @@ class FlutterLocalFileComparator extends FlutterGoldenFileComparator with LocalC
       }
       failureDiffs[expectation] = result;
     }
-    failureDiffs.forEach((String expectation, ComparisonResult result) async {
-      if (await skiaClient.isValidDigestForExpectation(expectation, golden.path))
-        generateFailureOutput(result, golden, basedir, key: expectation);
-    });
+
+    for (MapEntry<String, ComparisonResult> entry in failureDiffs.entries) {
+      if (await skiaClient.isValidDigestForExpectation(entry.key, golden.path))
+        generateFailureOutput(entry.value, golden, basedir, key: entry.key);
+    }
     return false;
   }
 }

packages/flutter_goldens/test/flutter_goldens_test.dart

@@ -741,6 +741,27 @@ void main() {
           isTrue,
         );
       });
+
+      test('compare properly awaits validation & output before failing.', () async {
+        final Completer<bool> completer = Completer<bool>();
+        when(mockSkiaClient.isValidDigestForExpectation(
+          '55109a4bed52acc780530f7a9aeff6c0',
+          'library.flutter.golden_test.1.png',
+        ))
+          .thenAnswer((_) => completer.future);
+        final Future<bool> result = comparator.compare(
+          Uint8List.fromList(_kFailPngBytes),
+          Uri.parse('flutter.golden_test.1.png'),
+        );
+        bool shouldThrow = true;
+        result.then((_) {
+          if (shouldThrow)
+            fail('Compare completed before validation completed!');
+        });
+        await Future<void>.value();
+        shouldThrow = false;
+        completer.complete(Future<bool>.value(false));
+      });
     });
   });
 }