Fix dartdoc_index_parsing_test.dart

Author: sigurdm

app/test/search/dartdoc_index_parsing_test.dart

@@ -39,8 +39,9 @@ void main() {
     }
 
     test('parse Dart SDK index.json', () async {
+      final channel = runtimeSdkVersion.endsWith('dev') ? 'dev' : 'stable';
       final file = await getCachedFile('dart-sdk-$runtimeSdkVersion.json',
-          'https://api.dart.dev/stable/$runtimeSdkVersion/index.json');
+          'https://api.dart.dev/$channel/$runtimeSdkVersion/index.json');
       final textContent = await file.readAsString();
       final index = DartdocIndex.parseJsonText(await file.readAsString());
       expect(index.entries, hasLength(greaterThan(10000)));