Added tool to easily check golden diffs locally. (#50654)

This addresses the problem where people feel like they have to upload a PR to get diffs from Skia Gold.  This should work for most workflows.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: gaaclarke

tools/compare_goldens/README.md

@@ -0,0 +1,37 @@
+# Compare Goldens
+
+This is a script that will let you check golden image diffs locally.
+
+The directories are scanned for png files that match in name, then the diff
+is written to `diff_<name of file>` in the CWD. This allows you to get
+results quicker than having to upload to skia gold.  By default it uses fuzzy
+RMSE to compare.
+
+## Usage
+
+```sh
+dart run compare_goldens <dir path> <dir path>
+```
+
+Here's the steps for using this with something like impeller golden tests:
+
+1) Checkout a base revision
+2) Build impeller_golden_tests
+3) Execute `impeller_golden_tests --working_dir=\<path a\>
+4) Checkout test revision
+5) Build impeller_golden_tests
+6) Execute `impeller_golden_tests --working_dir=\<path b\>
+7) Execute `compare_goldens \<path a\> \<path b\>
+
+## Requirements
+
+- ImageMagick is installed on $PATH
+
+## Testing
+
+To run the tests:
+
+```sh
+dart pub get
+find . -name "*_test.dart" | xargs -n 1 dart --enable-asserts
+```

tools/compare_goldens/bin/compare_goldens.dart

@@ -0,0 +1,10 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:io' show exitCode;
+import 'package:compare_goldens/compare_goldens.dart' as compare_goldens;
+
+void main(List<String> args) {
+  exitCode = compare_goldens.run(args);
+}

tools/compare_goldens/lib/compare_goldens.dart

@@ -0,0 +1,85 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:io';
+
+bool _hasCommandOnPath(String name) {
+  final ProcessResult result = Process.runSync('which', <String>[name]);
+  return result.exitCode == 0;
+}
+
+List<String> _findPairs(Set<String> as, Set<String> bs) {
+  final List<String> result  = <String>[];
+  for (final String a in as) {
+    if (bs.contains(a)) {
+      result.add(a);
+    } else {
+      print('Mix match file $a.');
+    }
+  }
+  for (final String b in bs) {
+    if (!as.contains(b)) {
+      print('Mix match file $b.');
+    }
+  }
+  return result;
+}
+
+String _basename(String path) {
+  return path.split(Platform.pathSeparator).last;
+}
+
+Set<String> _grabPngFilenames(Directory dir) {
+  return dir.listSync()
+    .map((FileSystemEntity e) => _basename(e.path))
+    .where((String e) => e.endsWith('.png'))
+    .toSet();
+}
+
+/// The main entry point to the tool, execute it like `main`. Returns the
+/// `exitCode`.
+int run(List<String> args) {
+  int returnCode = 0;
+  if (!_hasCommandOnPath('compare')) {
+    throw Exception(r'Could not find `compare` from ImageMagick on $PATH.');
+  }
+  if (args.length != 2) {
+    throw Exception('Usage: compare_goldens.dart <dir path> <dir path>');
+  }
+
+  final Directory dirA = Directory(args[0]);
+  if (!dirA.existsSync()) {
+    throw Exception('Unable to find $dirA');
+  }
+  final Directory dirB = Directory(args[1]);
+  if (!dirB.existsSync()) {
+    throw Exception('Unable to find $dirB');
+  }
+
+  final Set<String> filesA = _grabPngFilenames(dirA);
+  final Set<String> filesB = _grabPngFilenames(dirB);
+  final List<String> pairs = _findPairs(filesA, filesB);
+
+  if (filesA.length != pairs.length || filesB.length != pairs.length) {
+    returnCode = 1;
+  }
+
+  int count = 0;
+  for (final String name in pairs) {
+    count += 1;
+    final String pathA = <String>[dirA.path, name].join(Platform.pathSeparator);
+    final String pathB = <String>[dirB.path, name].join(Platform.pathSeparator);
+    final String output = 'diff_$name';
+    print('compare ($count / ${pairs.length}) $name');
+    final ProcessResult result = Process.runSync('compare',
+        <String>['-metric', 'RMSE', '-fuzz', '5%', pathA, pathB, output]);
+    if (result.exitCode != 0) {
+      print('DIFF FOUND: saved to $output');
+      returnCode = 1;
+    } else {
+      File(output).deleteSync();
+    }
+  }
+  return returnCode;
+}

tools/compare_goldens/pubspec.yaml

@@ -0,0 +1,3 @@
+name: compare_goldens
+environment:
+  sdk: '>=3.2.0-0 <4.0.0'

tools/compare_goldens/test/compare_goldens_test.dart

@@ -0,0 +1,5 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+void main() {}

tools/pub_get_offline.py

@@ -34,6 +34,7 @@
     os.path.join(ENGINE_DIR, 'tools', 'api_check'),
     os.path.join(ENGINE_DIR, 'tools', 'build_bucket_golden_scraper'),
     os.path.join(ENGINE_DIR, 'tools', 'clang_tidy'),
+    os.path.join(ENGINE_DIR, 'tools', 'compare_goldens'),
     os.path.join(ENGINE_DIR, 'tools', 'const_finder'),
     os.path.join(ENGINE_DIR, 'tools', 'engine_tool'),
     os.path.join(ENGINE_DIR, 'tools', 'gen_web_locale_keymap'),