Re-land Assert golden files use the right extension (#99082)

dev/automated_tests/flutter_test/flutter_gold_expectation.txt

 [0-9]+:[0-9]+ [+]0: Local passes non-existent baseline for new test, null expectation *
- *No expectations provided by Skia Gold for test: library.flutter.new_golden_test.1. This may be a new test. If this is an unexpected result, check https://flutter-gold.skia.org.
+ *No expectations provided by Skia Gold for test: library.flutter.new_golden_test.1.png. This may be a new test. If this is an unexpected result, check https://flutter-gold.skia.org.
  *Validate image output found at flutter/test/library/
 [0-9]+:[0-9]+ [+]1: Local passes non-existent baseline for new test, empty expectation *
- *No expectations provided by Skia Gold for test: library.flutter.new_golden_test.2. This may be a new test. If this is an unexpected result, check https://flutter-gold.skia.org.
+ *No expectations provided by Skia Gold for test: library.flutter.new_golden_test.2.png. This may be a new test. If this is an unexpected result, check https://flutter-gold.skia.org.
  *Validate image output found at flutter/test/library/
 [0-9]+:[0-9]+ [+]2: All tests passed! *

dev/automated_tests/flutter_test/flutter_gold_test.dart

@@ -38,7 +38,7 @@ void main() {
     expect(
       await comparator.compare(
         Uint8List.fromList(_kFailPngBytes),
-        Uri.parse('flutter.new_golden_test.1'),
+        Uri.parse('flutter.new_golden_test.1.png'),
       ),
       isTrue,
     );
@@ -48,7 +48,7 @@ void main() {
     expect(
       await comparator.compare(
         Uint8List.fromList(_kFailPngBytes),
-        Uri.parse('flutter.new_golden_test.2'),
+        Uri.parse('flutter.new_golden_test.2.png'),
       ),
       isTrue,
     );

packages/flutter_goldens/lib/flutter_goldens.dart

@@ -167,6 +167,12 @@ abstract class FlutterGoldenFileComparator extends GoldenFileComparator {
   /// Prepends the golden URL with the library name that encloses the current
   /// test.
   Uri _addPrefix(Uri golden) {
+    // Ensure the Uri ends in .png as the SkiaClient expects
+    assert(
+      golden.toString().split('.').last == 'png',
+      'Golden files in the Flutter framework must end with the file extension '
+      '.png.'
+    );
     final String prefix = basedir.pathSegments[basedir.pathSegments.length - 2];
     return Uri.parse('$prefix.$golden');
   }

packages/flutter_goldens/test/flutter_goldens_test.dart

@@ -481,6 +481,26 @@ void main() {
         );
       });
 
+      test('asserts .png format', () async {
+        await expectLater(
+          () async {
+            return comparator.compare(
+              Uint8List.fromList(_kTestPngBytes),
+              Uri.parse('flutter.golden_test.1'),
+            );
+          },
+          throwsA(
+            isA<AssertionError>().having((AssertionError error) => error.toString(),
+              'description',
+              contains(
+                'Golden files in the Flutter framework must end with the file '
+                'extension .png.'
+              ),
+            ),
+          ),
+        );
+      });
+
       test('calls init during compare', () {
         expect(fakeSkiaClient.initCalls, 0);
         comparator.compare(
@@ -579,6 +599,26 @@ void main() {
         );
       });
 
+      test('asserts .png format', () async {
+        await expectLater(
+          () async {
+            return comparator.compare(
+              Uint8List.fromList(_kTestPngBytes),
+              Uri.parse('flutter.golden_test.1'),
+            );
+          },
+          throwsA(
+            isA<AssertionError>().having((AssertionError error) => error.toString(),
+              'description',
+              contains(
+                'Golden files in the Flutter framework must end with the file '
+                'extension .png.'
+              ),
+            ),
+          ),
+        );
+      });
+
       test('calls init during compare', () {
         expect(fakeSkiaClient.tryInitCalls, 0);
         comparator.compare(
@@ -745,6 +785,26 @@ void main() {
         fakeSkiaClient.cleanTestNameValues['library.flutter.golden_test.1.png'] = 'flutter.golden_test.1';
       });
 
+      test('asserts .png format', () async {
+        await expectLater(
+          () async {
+            return comparator.compare(
+              Uint8List.fromList(_kTestPngBytes),
+              Uri.parse('flutter.golden_test.1'),
+            );
+          },
+          throwsA(
+            isA<AssertionError>().having((AssertionError error) => error.toString(),
+              'description',
+              contains(
+                'Golden files in the Flutter framework must end with the file '
+                'extension .png.'
+              ),
+            ),
+          ),
+        );
+      });
+
       test('passes when bytes match', () async {
         expect(
           await comparator.compare(