Tiny fix about outdated message (#114391)

Author: fzyzcjy

packages/flutter_tools/lib/src/commands/screenshot.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:meta/meta.dart';
 import 'package:vm_service/vm_service.dart' as vm_service;
 
 import '../base/common.dart';
@@ -168,7 +169,7 @@ class ScreenshotCommand extends FlutterCommand {
     sink.add(base64.decode(skp.json?['skp'] as String));
     await sink.close();
     _showOutputFileInfo(outputFile);
-    _ensureOutputIsNotJsonRpcError(outputFile);
+    ensureOutputIsNotJsonRpcError(outputFile);
     return true;
   }
 
@@ -192,7 +193,7 @@ class ScreenshotCommand extends FlutterCommand {
     sink.add(base64.decode(response.json?['screenshot'] as String));
     await sink.close();
     _showOutputFileInfo(outputFile);
-    _ensureOutputIsNotJsonRpcError(outputFile);
+    ensureOutputIsNotJsonRpcError(outputFile);
     return true;
   }
 
@@ -205,15 +206,16 @@ class ScreenshotCommand extends FlutterCommand {
     }
   }
 
-  void _ensureOutputIsNotJsonRpcError(File outputFile) {
+  @visibleForTesting
+  static void ensureOutputIsNotJsonRpcError(File outputFile) {
     if (outputFile.lengthSync() >= 1000) {
       return;
     }
     final String content = outputFile.readAsStringSync(
       encoding: const AsciiCodec(allowInvalid: true),
     );
     if (content.startsWith('{"jsonrpc":"2.0", "error"')) {
-      throwToolExit('It appears the output file contains an error message, not valid skia output.');
+      throwToolExit('It appears the output file contains an error message, not valid output.');
     }
   }
 

packages/flutter_tools/test/commands.shard/hermetic/screenshot_command_test.dart

@@ -106,4 +106,24 @@ void main() {
               message: 'File was not created, ensure path is valid'));
     });
   });
+
+  group('Screenshot output validation', () {
+    testWithoutContext('successful', () async {
+      final MemoryFileSystem fs = MemoryFileSystem.test();
+      fs.file('test.png').createSync();
+
+      expect(() => ScreenshotCommand.ensureOutputIsNotJsonRpcError(fs.file('test.png')),
+          returnsNormally);
+    });
+
+    testWithoutContext('failed', () async {
+      final MemoryFileSystem fs = MemoryFileSystem.test();
+      fs.file('test.png').writeAsStringSync('{"jsonrpc":"2.0", "error":"something"}');
+
+      expect(
+          () => ScreenshotCommand.ensureOutputIsNotJsonRpcError(fs.file('test.png')),
+          throwsToolExit(
+              message: 'It appears the output file contains an error message, not valid output.'));
+    });
+  });
 }