[ci] Manual roll Flutter to c246ecd (84 revisions) + fixes (flutter#6863)

Updates the flutter master version and adds fixes for broken stuff.

* [flutter_markdown] Updates a test image used as a golden file.
* ~~[google_maps_flutter_android]~~ Flaky test/fixed OOB.

## Issues

* Completes flutter/packages#6861

Author: ditman

.ci/flutter_master.version

@@ -1 +1 @@
-c85fa6abf4ffe253109bc5cac762e46d1853d325
+c246ecdf8e8d564410698e86cc07e853269ebab8

packages/flutter_markdown/test/assets/images/golden/image_test/custom_builder_asset_logo.png

@@ -13,6 +13,8 @@ import 'package:flutter_test/flutter_test.dart';
 import 'image_test_mocks.dart';
 import 'utils.dart';
 
+bool isRunningInStable = io.Platform.environment['CHANNEL'] == 'stable';
+
 void main() => defineTests();
 
 void defineTests() {
@@ -167,8 +169,9 @@ void defineTests() {
 
         await expectLater(
             find.byType(Container),
-            matchesGoldenFile(
-                'assets/images/golden/image_test/resource_asset_logo.png'));
+            matchesGoldenFile(isRunningInStable
+                ? 'assets/images/golden/image_test/resource_asset_logo_old.png'
+                : 'assets/images/golden/image_test/resource_asset_logo.png'));
       },
       skip: kIsWeb, // Goldens are platform-specific.
     );
@@ -413,8 +416,9 @@ void defineTests() {
 
         await expectLater(
             find.byType(Container),
-            matchesGoldenFile(
-                'assets/images/golden/image_test/custom_builder_asset_logo.png'));
+            matchesGoldenFile(isRunningInStable
+                ? 'assets/images/golden/image_test/custom_builder_asset_logo_old.png'
+                : 'assets/images/golden/image_test/custom_builder_asset_logo.png'));
       },
       skip: kIsWeb, // Goldens are platform-specific.
     );