Convert SkiaException to TestFailure on post-submit. (#162623)

Fixes https://github.com/flutter/flutter/issues/162621.

I believe this has no other implications, other than it's possible to
catch `TestFailure` where it should be catchable today.

packages/flutter_goldens/lib/flutter_goldens.dart

@@ -320,7 +320,17 @@ class FlutterPostSubmitFileComparator extends FlutterGoldenFileComparator {
     golden = _addPrefix(golden);
     await update(golden, imageBytes);
     final File goldenFile = getGoldenFile(golden);
-    return skiaClient.imgtestAdd(golden.path, goldenFile);
+    try {
+      return await skiaClient.imgtestAdd(golden.path, goldenFile);
+    } on SkiaException catch (e) {
+      // Convert SkiaException -> TestFailure so that this class implements the
+      // contract of GoldenFileComparator, and matchesGoldenFile() converts the
+      // TestFailure into a standard reported test error (with a better stack
+      // trace, for example).
+      //
+      // https://github.com/flutter/flutter/issues/162621
+      throw TestFailure('$e');
+    }
   }
 
   /// Decides based on the current environment if goldens tests should be

packages/flutter_goldens/test/flutter_goldens_test.dart

@@ -853,6 +853,43 @@ void main() {
         );
         expect(fakeSkiaClient.initCalls, 0);
       });
+
+      test('reports a failure as a TestFailure', () async {
+        final List<String> log = <String>[];
+        final MemoryFileSystem fs = MemoryFileSystem();
+        final FakePlatform platform = FakePlatform(
+          environment: <String, String>{'FLUTTER_ROOT': _kFlutterRoot},
+          operatingSystem: 'macos',
+        );
+        fs.directory(_kFlutterRoot).createSync(recursive: true);
+        final Directory basedir = fs.directory('flutter/test/library/')
+          ..createSync(recursive: true);
+        final FlutterGoldenFileComparator comparator = FlutterPostSubmitFileComparator(
+          basedir.uri,
+          ThrowsOnImgTestAddSkiaClient(
+            message: 'Skia Gold received an unapproved image in post-submit',
+          ),
+          fs: fs,
+          platform: platform,
+          log: log.add,
+        );
+        await expectLater(
+          () async {
+            return comparator.compare(
+              Uint8List.fromList(_kTestPngBytes),
+              Uri.parse('flutter.golden_test.1.png'),
+            );
+          },
+          throwsA(
+            isA<TestFailure>().having(
+              (TestFailure error) => error.toString(),
+              'description',
+              contains('Skia Gold received an unapproved image in post-submit'),
+            ),
+          ),
+        );
+        expect(log, isEmpty);
+      });
     });
 
     group('Pre-Submit', () {
@@ -1210,6 +1247,21 @@ class FakeSkiaGoldClient extends Fake implements SkiaGoldClient {
   String cleanTestName(String fileName) => cleanTestNameValues[fileName] ?? '';
 }
 
+class ThrowsOnImgTestAddSkiaClient extends Fake implements SkiaGoldClient {
+  ThrowsOnImgTestAddSkiaClient({required this.message});
+  final String message;
+
+  @override
+  Future<void> imgtestInit() async {
+    // Assume this function works.
+  }
+
+  @override
+  Future<bool> imgtestAdd(String testName, File goldenFile) {
+    throw SkiaException(message);
+  }
+}
+
 class FakeLocalFileComparator extends Fake implements LocalFileComparator {
   @override
   late Uri basedir;