Add Flutter-repo-specific golden comparator (#17205)

In order to avoid checking binaries into flutter/flutter,
this comparator can be used to retrieve golden files from
a sibling flutter/goldens repository.

https://github.com/flutter/flutter/issues/16859

bin/internal/goldens.version

+0ea80e6a0147f1a3a59ff57f460a3f038a0d2748

dev/tools/vitool/pubspec.yaml

@@ -64,4 +64,4 @@ dev_dependencies:
   web_socket_channel: 1.0.7 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   yaml: 2.1.13 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
 
-# PUBSPEC CHECKSUM: 2b15
+# PUBSPEC CHECKSUM: f3fb

examples/flutter_gallery/pubspec.yaml

@@ -29,6 +29,8 @@ dev_dependencies:
     sdk: flutter
   flutter_driver:
     sdk: flutter
+  flutter_goldens:
+    sdk: flutter
 
   args: 1.4.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
   async: 2.0.6 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
@@ -209,4 +211,4 @@ flutter:
         - asset: packages/flutter_gallery_assets/fonts/GoogleSansDisplay-Regular.ttf
           weight: 400
 
-# PUBSPEC CHECKSUM: 50c7
+# PUBSPEC CHECKSUM: 7b1f

examples/flutter_gallery/test/flutter_test_config.dart

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+export 'package:flutter_goldens/flutter_goldens.dart' show main;

packages/flutter/pubspec.yaml

@@ -16,6 +16,8 @@ dependencies:
 dev_dependencies:
   flutter_test:
     sdk: flutter
+  flutter_goldens:
+    sdk: flutter
   mockito: 3.0.0-alpha+3
 
   args: 1.4.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
@@ -67,4 +69,4 @@ dev_dependencies:
 environment:
   sdk: '>=1.19.0 <2.0.0'
 
-# PUBSPEC CHECKSUM: a5bf
+# PUBSPEC CHECKSUM: 8817

packages/flutter/test/flutter_test_config.dart

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+export 'package:flutter_goldens/flutter_goldens.dart' show main;

packages/flutter_goldens/lib/flutter_goldens.dart

+// Copyright 2018 The Chromium 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:async';
+import 'dart:io' as io;
+import 'dart:typed_data';
+
+import 'package:collection/collection.dart';
+import 'package:file/file.dart';
+import 'package:file/local.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:meta/meta.dart';
+import 'package:platform/platform.dart';
+import 'package:process/process.dart';
+
+const String _kFlutterRootKey = 'FLUTTER_ROOT';
+
+/// Main method that can be used in a `flutter_test_config.dart` file to set
+/// [goldenFileComparator] to an instance of [FlutterGoldenFileComparator] that
+/// works for the current test.
+Future<void> main(FutureOr<void> testMain()) async {
+  goldenFileComparator = await FlutterGoldenFileComparator.fromDefaultComparator();
+  await testMain();
+}
+
+/// A golden file comparator specific to the `flutter/flutter` repository.
+///
+/// Within the https://github.com/flutter/flutter repository, it's important
+/// not to check-in binaries in order to keep the size of the repository to a
+/// minimum. To satisfy this requirement, this comparator retrieves the golden
+/// files from a sibling repository, `flutter/goldens`.
+///
+/// This comparator will locally clone the `flutter/goldens` repository into
+/// the `$FLUTTER_ROOT/bin/cache/pkg/goldens` folder, then perform the comparison against
+/// the files therein.
+class FlutterGoldenFileComparator implements GoldenFileComparator {
+  @visibleForTesting
+  FlutterGoldenFileComparator(
+    this.goldens,
+    this.basedir, {
+    this.fs: const LocalFileSystem(),
+  });
+
+  final GoldensClient goldens;
+  final Uri basedir;
+  final FileSystem fs;
+
+  /// Creates a new [FlutterGoldenFileComparator] that mirrors the relative
+  /// path resolution of the default [goldenFileComparator].
+  ///
+  /// By the time the future completes, the clone of the `flutter/goldens`
+  /// repository is guaranteed to be ready use.
+  static Future<FlutterGoldenFileComparator> fromDefaultComparator() async {
+    final LocalFileComparator defaultComparator = goldenFileComparator;
+    final GoldensClient goldens = new GoldensClient();
+    await goldens.prepare();
+    return new FlutterGoldenFileComparator(goldens, defaultComparator.basedir);
+  }
+
+  @override
+  Future<bool> compare(Uint8List imageBytes, Uri golden) async {
+    final File goldenFile = _getGoldenFile(golden);
+    if (!goldenFile.existsSync()) {
+      throw new TestFailure('Could not be compared against non-existent file: "$golden"');
+    }
+    final List<int> goldenBytes = await goldenFile.readAsBytes();
+    // TODO(tvolkert): Improve the intelligence of this comparison.
+    return const ListEquality<int>().equals(goldenBytes, imageBytes);
+  }
+
+  @override
+  Future<void> update(Uri golden, Uint8List imageBytes) async {
+    final File goldenFile = _getGoldenFile(golden);
+    await goldenFile.parent.create(recursive: true);
+    await goldenFile.writeAsBytes(imageBytes, flush: true);
+  }
+
+  File _getGoldenFile(Uri uri) {
+    final File relativeFile = fs.file(uri);
+    final Directory testDirectory = fs.directory(basedir);
+    final String relativeBase = fs.path.relative(testDirectory.path, from: goldens.flutterRoot.path);
+    return goldens.repositoryRoot.childDirectory(relativeBase).childFile(relativeFile.path);
+  }
+}
+
+/// A class that represents a clone of the https://github.com/flutter/goldens
+/// repository, nested within the `bin/cache` directory of the caller's Flutter
+/// repository.
+@visibleForTesting
+class GoldensClient {
+  GoldensClient({
+    this.fs: const LocalFileSystem(),
+    this.platform: const LocalPlatform(),
+    this.process: const LocalProcessManager(),
+  });
+
+  final FileSystem fs;
+  final Platform platform;
+  final ProcessManager process;
+
+  RandomAccessFile _lock;
+
+  Directory get flutterRoot => fs.directory(platform.environment[_kFlutterRootKey]);
+
+  Directory get repositoryRoot => flutterRoot.childDirectory(fs.path.join('bin', 'cache', 'pkg', 'goldens'));
+
+  /// Prepares the local clone of the `flutter/goldens` repository for golden
+  /// file testing.
+  ///
+  /// This ensures that the goldens repository has been cloned into its
+  /// expected location within `bin/cache` and that it is synced to the Git
+  /// revision specified in `bin/internal/goldens.version`.
+  ///
+  /// While this is preparing the repository, it obtains a file lock such that
+  /// [GoldensClient] instances in other processes or isolates will not
+  /// duplicate the work that this is doing.
+  Future<void> prepare() async {
+    final String goldensCommit = await _getGoldensCommit();
+    String currentCommit = await _getCurrentCommit();
+    if (currentCommit != goldensCommit) {
+      await _obtainLock();
+      try {
+        // Check the current commit again now that we have the lock.
+        currentCommit = await _getCurrentCommit();
+        if (currentCommit != goldensCommit) {
+          if (currentCommit == null) {
+            await _initRepository();
+          }
+          await _syncTo(goldensCommit);
+        }
+      } finally {
+        await _releaseLock();
+      }
+    }
+  }
+
+  Future<String> _getGoldensCommit() async {
+    final File versionFile = flutterRoot.childFile(fs.path.join('bin', 'internal', 'goldens.version'));
+    return (await versionFile.readAsString()).trim();
+  }
+
+  Future<String> _getCurrentCommit() async {
+    if (!repositoryRoot.existsSync()) {
+      return null;
+    } else {
+      final io.ProcessResult revParse = await process.run(
+        <String>['git', 'rev-parse', 'HEAD'],
+        workingDirectory: repositoryRoot.path,
+      );
+      return revParse.exitCode == 0 ? revParse.stdout.trim() : null;
+    }
+  }
+
+  Future<void> _initRepository() async {
+    await repositoryRoot.create(recursive: true);
+    await _runCommands(
+      <String>[
+        'git init',
+        'git remote add upstream https://github.com/flutter/goldens.git',
+      ],
+      workingDirectory: repositoryRoot,
+    );
+  }
+
+  Future<void> _syncTo(String commit) async {
+    await _runCommands(
+      <String>[
+        'git pull upstream master',
+        'git fetch upstream $commit',
+        'git reset --hard FETCH_HEAD',
+      ],
+      workingDirectory: repositoryRoot,
+    );
+  }
+
+  Future<void> _runCommands(
+    List<String> commands, {
+    Directory workingDirectory,
+  }) async {
+    for (String command in commands) {
+      final List<String> parts = command.split(' ');
+      final io.ProcessResult result = await process.run(
+        parts,
+        workingDirectory: workingDirectory?.path,
+      );
+      if (result.exitCode != 0) {
+        throw new NonZeroExitCode(result.exitCode, result.stderr);
+      }
+    }
+  }
+
+  Future<void> _obtainLock() async {
+    final File lockFile = flutterRoot.childFile(fs.path.join('bin', 'cache', 'goldens.lockfile'));
+    await lockFile.create(recursive: true);
+    _lock = await lockFile.open(mode: io.FileMode.WRITE);
+    await _lock.lock(io.FileLock.BLOCKING_EXCLUSIVE);
+  }
+
+  Future<void> _releaseLock() async {
+    await _lock.close();
+    _lock = null;
+  }
+}
+
+/// Exception that signals a process' exit with a non-zero exit code.
+class NonZeroExitCode implements Exception {
+  const NonZeroExitCode(this.exitCode, this.stderr) : assert(exitCode != 0);
+
+  final int exitCode;
+  final String stderr;
+
+  @override
+  String toString() {
+    return 'Exit code $exitCode: $stderr';
+  }
+}

packages/flutter_goldens/pubspec.yaml

+name: flutter_goldens
+
+dependencies:
+  # To update these, use "flutter update-packages --force-upgrade".
+  flutter_test:
+    sdk: flutter
+  collection: 1.14.6
+  file: 5.0.0
+  meta: 1.1.2
+  platform: 2.1.2
+  process: 3.0.1
+
+  args: 1.4.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  async: 2.0.6 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  barback: 0.15.2+15 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  boolean_selector: 1.0.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  charcode: 1.1.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  convert: 2.0.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  crypto: 2.0.2+1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  csslib: 0.14.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  glob: 1.1.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  html: 0.13.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  http: 0.11.3+16 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  http_multi_server: 2.0.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  http_parser: 3.1.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  intl: 0.15.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  io: 0.3.2+1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  js: 0.6.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  logging: 0.11.3+1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  matcher: 0.12.1+4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  mime: 0.9.6 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  multi_server_socket: 1.0.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  node_preamble: 1.4.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  package_config: 1.0.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  package_resolver: 1.0.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  path: 1.5.1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  plugin: 0.2.0+2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  pool: 1.3.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  pub_semver: 1.3.6 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  quiver: 0.29.0+1 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  shelf: 0.7.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  shelf_packages_handler: 1.0.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  shelf_static: 0.2.7 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  shelf_web_socket: 0.2.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  source_map_stack_trace: 1.1.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  source_maps: 0.10.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  source_span: 1.4.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  stack_trace: 1.9.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  stream_channel: 1.6.4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  string_scanner: 1.0.2 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  term_glyph: 1.0.0 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  test: 0.12.34 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  typed_data: 1.1.5 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  utf: 0.9.0+4 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  vector_math: 2.0.6 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  watcher: 0.9.7+7 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  web_socket_channel: 1.0.7 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+  yaml: 2.1.13 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
+
+dev_dependencies:
+  mockito: 3.0.0-alpha+3
+
+environment:
+  sdk: '>=1.19.0 <2.0.0'
+
+# PUBSPEC CHECKSUM: 4bf9

packages/flutter_goldens/test/flutter_goldens_test.dart

+// Copyright 2018 The Chromium 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' as io;
+import 'dart:typed_data';
+
+import 'package:file/file.dart';
+import 'package:file/memory.dart';
+import 'package:flutter_goldens/flutter_goldens.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:mockito/mockito.dart';
+import 'package:platform/platform.dart';
+import 'package:process/process.dart';
+
+const String _kFlutterRoot = '/flutter';
+const String _kRepositoryRoot = '$_kFlutterRoot/bin/cache/pkg/goldens';
+const String _kVersionFile = '$_kFlutterRoot/bin/internal/goldens.version';
+const String _kGoldensVersion = '123456abcdef';
+
+void main() {
+  MemoryFileSystem fs;
+  FakePlatform platform;
+  MockProcessManager process;
+
+  setUp(() {
+    fs = new MemoryFileSystem();
+    platform = new FakePlatform(environment: <String, String>{'FLUTTER_ROOT': _kFlutterRoot});
+    process = new MockProcessManager();
+    fs.directory(_kFlutterRoot).createSync(recursive: true);
+    fs.directory(_kRepositoryRoot).createSync(recursive: true);
+    fs.file(_kVersionFile).createSync(recursive: true);
+    fs.file(_kVersionFile).writeAsStringSync(_kGoldensVersion);
+  });
+
+  group('GoldensClient', () {
+    GoldensClient goldens;
+
+    setUp(() {
+      goldens = new GoldensClient(
+        fs: fs,
+        platform: platform,
+        process: process,
+      );
+    });
+
+    group('prepare', () {
+      test('performs minimal work if versions match', () async {
+        when(process.run(typed(captureAny), workingDirectory: typed(captureAny, named: 'workingDirectory')))
+            .thenAnswer((_) => new Future<io.ProcessResult>.value(io.ProcessResult(123, 0, _kGoldensVersion, '')));
+        await goldens.prepare();
+
+        // Verify that we only spawned `git rev-parse HEAD`
+        final VerificationResult verifyProcessRun =
+            verify(process.run(typed(captureAny), workingDirectory: typed(captureAny, named: 'workingDirectory')));
+        verifyProcessRun.called(1);
+        expect(verifyProcessRun.captured.first, <String>['git', 'rev-parse', 'HEAD']);
+        expect(verifyProcessRun.captured.last, _kRepositoryRoot);
+      });
+    });
+  });
+
+  group('FlutterGoldenFileComparator', () {
+    GoldensClient goldens;
+    MemoryFileSystem fs;
+    FlutterGoldenFileComparator comparator;
+
+    setUp(() {
+      goldens = new MockGoldensClient();
+      fs = new MemoryFileSystem();
+      final Directory flutterRoot = fs.directory('/path/to/flutter')..createSync(recursive: true);
+      final Directory goldensRoot = flutterRoot.childDirectory('bin/cache/goldens')..createSync(recursive: true);
+      final Directory testDirectory = flutterRoot.childDirectory('test/foo/bar')..createSync(recursive: true);
+      comparator = new FlutterGoldenFileComparator(goldens, new Uri.directory(testDirectory.path), fs: fs);
+      when(goldens.flutterRoot).thenReturn(flutterRoot);
+      when(goldens.repositoryRoot).thenReturn(goldensRoot);
+    });
+
+    group('compare', () {
+      test('throws if golden file is not found', () async {
+        try {
+          await comparator.compare(new Uint8List.fromList(<int>[1, 2, 3]), Uri.parse('test.png'));
+          fail('TestFailure expected but not thrown');
+        } on TestFailure catch (error) {
+          expect(error.message, contains('Could not be compared against non-existent file'));
+        }
+      });
+
+      test('returns false if golden bytes do not match', () async {
+        final File goldenFile = fs.file('/path/to/flutter/bin/cache/goldens/test/foo/bar/test.png')
+          ..createSync(recursive: true);
+        goldenFile.writeAsBytesSync(<int>[4, 5, 6], flush: true);
+        final bool result = await comparator.compare(new Uint8List.fromList(<int>[1, 2, 3]), Uri.parse('test.png'));
+        expect(result, isFalse);
+      });
+
+      test('returns true if golden bytes match', () async {
+        final File goldenFile = fs.file('/path/to/flutter/bin/cache/goldens/test/foo/bar/test.png')
+          ..createSync(recursive: true);
+        goldenFile.writeAsBytesSync(<int>[1, 2, 3], flush: true);
+        final bool result = await comparator.compare(new Uint8List.fromList(<int>[1, 2, 3]), Uri.parse('test.png'));
+        expect(result, isTrue);
+      });
+    });
+
+    group('update', () {
+      test('creates golden file if it does not already exist', () async {
+        final File goldenFile = fs.file('/path/to/flutter/bin/cache/goldens/test/foo/bar/test.png');
+        expect(goldenFile.existsSync(), isFalse);
+        await comparator.update(Uri.parse('test.png'), new Uint8List.fromList(<int>[1, 2, 3]));
+        expect(goldenFile.existsSync(), isTrue);
+        expect(goldenFile.readAsBytesSync(), <int>[1, 2, 3]);
+      });
+
+      test('overwrites golden bytes if golden file already exist', () async {
+        final File goldenFile = fs.file('/path/to/flutter/bin/cache/goldens/test/foo/bar/test.png')
+          ..createSync(recursive: true);
+        goldenFile.writeAsBytesSync(<int>[4, 5, 6], flush: true);
+        await comparator.update(Uri.parse('test.png'), new Uint8List.fromList(<int>[1, 2, 3]));
+        expect(goldenFile.readAsBytesSync(), <int>[1, 2, 3]);
+      });
+    });
+  });
+}
+
+class MockProcessManager extends Mock implements ProcessManager {}
+class MockGoldensClient extends Mock implements GoldensClient {}

packages/flutter_test/lib/src/goldens.dart

@@ -66,7 +66,11 @@ abstract class GoldenFileComparator {
 ///
 ///  * [flutter_test] for more information about how to configure tests at the
 ///    directory-level.
-GoldenFileComparator goldenFileComparator = const _UninitializedComparator();
+GoldenFileComparator _goldenFileComparator = const _UninitializedComparator();
+GoldenFileComparator get goldenFileComparator => _goldenFileComparator;
+set goldenFileComparator(GoldenFileComparator comparator) {
+  _goldenFileComparator = comparator ?? const _UninitializedComparator();
+}
 
 /// Whether golden files should be automatically updated during tests rather
 /// than compared to the image bytes recorded by the tests.
@@ -120,15 +124,19 @@ class LocalFileComparator implements GoldenFileComparator {
   /// directory in which [testFile] resides.
   ///
   /// The [testFile] URI must represent a file.
-  LocalFileComparator(Uri testFile)
+  LocalFileComparator(Uri testFile, {path.Style pathStyle})
       : assert(testFile.scheme == 'file'),
-        basedir = new Uri.directory(_path.dirname(_path.fromUri(testFile)));
+        basedir = _getBasedir(testFile, pathStyle),
+        _path = _getPath(pathStyle);
 
-  // Due to https://github.com/flutter/flutter/issues/17118, we need to
-  // explicitly set the path style.
-  static final path.Context _path = new path.Context(style: Platform.isWindows
-      ? path.Style.windows
-      : path.Style.posix);
+  static path.Context _getPath(path.Style style) {
+    return new path.Context(style: style ?? path.Style.platform);
+  }
+
+  static Uri _getBasedir(Uri testFile, path.Style pathStyle) {
+    final path.Context context = _getPath(pathStyle);
+    return context.toUri(context.dirname(context.fromUri(testFile)) + context.separator);
+  }
 
   /// The directory in which the test was loaded.
   ///
@@ -136,6 +144,11 @@ class LocalFileComparator implements GoldenFileComparator {
   /// directory.
   final Uri basedir;
 
+  /// Path context exists as an instance variable rather than just using the
+  /// system path context in order to support testing, where we can spoof the
+  /// platform to test behaviors with arbitrary path styles.
+  final path.Context _path;
+
   @override
   Future<bool> compare(Uint8List imageBytes, Uri golden) async {
     final File goldenFile = _getFile(golden);
@@ -149,11 +162,12 @@ class LocalFileComparator implements GoldenFileComparator {
   @override
   Future<void> update(Uri golden, Uint8List imageBytes) async {
     final File goldenFile = _getFile(golden);
+    await goldenFile.parent.create(recursive: true);
     await goldenFile.writeAsBytes(imageBytes, flush: true);
   }
 
   File _getFile(Uri golden) {
-    return new File(_path.join(_path.fromUri(basedir), golden.path));
+    return new File(_path.join(_path.fromUri(basedir), _path.fromUri(golden.path)));
   }
 
   static bool _areListsEqual<T>(List<T> list1, List<T> list2) {

packages/flutter_test/test/goldens_test.dart

@@ -24,6 +24,16 @@ void main() {
     fs = new MemoryFileSystem(style: style);
   });
 
+  /// Converts posix-style paths to the style associated with [fs].
+  ///
+  /// This allows us to deal in posix-style paths in the tests.
+  String fix(String path) {
+    if (path.startsWith('/')) {
+      path = '${fs.style.drive}$path';
+    }
+    return path.replaceAll('/', fs.path.separator);
+  }
+
   void test(String description, FutureOr<void> body()) {
     test_package.test(description, () {
       return io.IOOverrides.runZoned(
@@ -59,18 +69,18 @@ void main() {
     LocalFileComparator comparator;
 
     setUp(() {
-      comparator = new LocalFileComparator(new Uri.file('/golden_test.dart'));
+      comparator = new LocalFileComparator(fs.file(fix('/golden_test.dart')).uri, pathStyle: fs.path.style);
     });
 
     test('calculates basedir correctly', () {
-      expect(comparator.basedir, new Uri.file('/'));
-      comparator = new LocalFileComparator(new Uri.file('/foo/bar/golden_test.dart'));
-      expect(comparator.basedir, new Uri.file('/foo/bar/'));
+      expect(comparator.basedir, fs.file(fix('/')).uri);
+      comparator = new LocalFileComparator(fs.file(fix('/foo/bar/golden_test.dart')).uri, pathStyle: fs.path.style);
+      expect(comparator.basedir, fs.directory(fix('/foo/bar/')).uri);
     });
 
     group('compare', () {
       Future<bool> doComparison([String golden = 'golden.png']) {
-        final Uri uri = new Uri.file(golden);
+        final Uri uri = fs.file(fix(golden)).uri;
         return comparator.compare(
           new Uint8List.fromList(_kExpectedBytes),
           uri,
@@ -79,13 +89,13 @@ void main() {
 
       group('succeeds', () {
         test('when golden file is in same folder as test', () async {
-          fs.file('/golden.png').writeAsBytesSync(_kExpectedBytes);
+          fs.file(fix('/golden.png')).writeAsBytesSync(_kExpectedBytes);
           final bool success = await doComparison();
           expect(success, isTrue);
         });
 
         test('when golden file is in subfolder of test', () async {
-          fs.file('/sub/foo.png')
+          fs.file(fix('/sub/foo.png'))
             ..createSync(recursive: true)
             ..writeAsBytesSync(_kExpectedBytes);
           final bool success = await doComparison('sub/foo.png');
@@ -100,32 +110,32 @@ void main() {
         });
 
         test('when golden bytes are leading subset of image bytes', () async {
-          fs.file('/golden.png').writeAsBytesSync(<int>[1, 2]);
+          fs.file(fix('/golden.png')).writeAsBytesSync(<int>[1, 2]);
           expect(await doComparison(), isFalse);
         });
 
         test('when golden bytes are leading superset of image bytes', () async {
-          fs.file('/golden.png').writeAsBytesSync(<int>[1, 2, 3, 4]);
+          fs.file(fix('/golden.png')).writeAsBytesSync(<int>[1, 2, 3, 4]);
           expect(await doComparison(), isFalse);
         });
 
         test('when golden bytes are trailing subset of image bytes', () async {
-          fs.file('/golden.png').writeAsBytesSync(<int>[2, 3]);
+          fs.file(fix('/golden.png')).writeAsBytesSync(<int>[2, 3]);
           expect(await doComparison(), isFalse);
         });
 
         test('when golden bytes are trailing superset of image bytes', () async {
-          fs.file('/golden.png').writeAsBytesSync(<int>[0, 1, 2, 3]);
+          fs.file(fix('/golden.png')).writeAsBytesSync(<int>[0, 1, 2, 3]);
           expect(await doComparison(), isFalse);
         });
 
         test('when golden bytes are disjoint from image bytes', () async {
-          fs.file('/golden.png').writeAsBytesSync(<int>[4, 5, 6]);
+          fs.file(fix('/golden.png')).writeAsBytesSync(<int>[4, 5, 6]);
           expect(await doComparison(), isFalse);
         });
 
         test('when golden bytes are empty', () async {
-          fs.file('/golden.png').writeAsBytesSync(<int>[]);
+          fs.file(fix('/golden.png')).writeAsBytesSync(<int>[]);
           expect(await doComparison(), isFalse);
         });
       });
@@ -133,18 +143,18 @@ void main() {
 
     group('update', () {
       test('updates existing file', () async {
-        fs.file('/golden.png').writeAsBytesSync(_kExpectedBytes);
+        fs.file(fix('/golden.png')).writeAsBytesSync(_kExpectedBytes);
         const List<int> newBytes = const <int>[11, 12, 13];
-        await comparator.update(new Uri.file('golden.png'), new Uint8List.fromList(newBytes));
-        expect(fs.file('/golden.png').readAsBytesSync(), newBytes);
+        await comparator.update(fs.file('golden.png').uri, new Uint8List.fromList(newBytes));
+        expect(fs.file(fix('/golden.png')).readAsBytesSync(), newBytes);
       });
 
       test('creates non-existent file', () async {
-        expect(fs.file('/foo.png').existsSync(), isFalse);
+        expect(fs.file(fix('/foo.png')).existsSync(), isFalse);
         const List<int> newBytes = const <int>[11, 12, 13];
-        await comparator.update(new Uri.file('foo.png'), new Uint8List.fromList(newBytes));
-        expect(fs.file('/foo.png').existsSync(), isTrue);
-        expect(fs.file('/foo.png').readAsBytesSync(), newBytes);
+        await comparator.update(fs.file('foo.png').uri, new Uint8List.fromList(newBytes));
+        expect(fs.file(fix('/foo.png')).existsSync(), isTrue);
+        expect(fs.file(fix('/foo.png')).readAsBytesSync(), newBytes);
       });
     });
   });

packages/flutter_tools/lib/src/test/flutter_platform.dart

@@ -685,9 +685,7 @@ void main() {
     );
     if (testConfigFile != null) {
       buffer.write('''
-    return () {
-      test_config.main(test.main);
-    };
+    return () => test_config.main(test.main);
 ''');
     } else {
       buffer.write('''