Add path_provider for elinux (#12)

Fixed #9

packages/path_provider/example/integration_test/path_provider_test.dart

@@ -0,0 +1,60 @@
+// 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';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:integration_test/integration_test.dart';
+import 'package:path_provider_elinux/path_provider_elinux.dart';
+
+void main() {
+  IntegrationTestWidgetsFlutterBinding.ensureInitialized();
+
+  testWidgets('getTemporaryDirectory', (WidgetTester tester) async {
+    final PathProviderELinux provider = PathProviderELinux();
+    final String? result = await provider.getTemporaryPath();
+    _verifySampleFile(result, 'temporaryDirectory');
+  });
+
+  testWidgets('getDownloadDirectory', (WidgetTester tester) async {
+    if (!Platform.isLinux) {
+      return;
+    }
+    final PathProviderELinux provider = PathProviderELinux();
+    final String? result = await provider.getDownloadsPath();
+    _verifySampleFile(result, 'downloadDirectory');
+  });
+
+  testWidgets('getApplicationDocumentsDirectory', (WidgetTester tester) async {
+    final PathProviderELinux provider = PathProviderELinux();
+    final String? result = await provider.getApplicationDocumentsPath();
+    _verifySampleFile(result, 'applicationDocuments');
+  });
+
+  testWidgets('getApplicationSupportDirectory', (WidgetTester tester) async {
+    final PathProviderELinux provider = PathProviderELinux();
+    final String? result = await provider.getApplicationSupportPath();
+    _verifySampleFile(result, 'applicationSupport');
+  });
+}
+
+/// Verify a file called [name] in [directoryPath] by recreating it with test
+/// contents when necessary.
+void _verifySampleFile(String? directoryPath, String name) {
+  expect(directoryPath, isNotNull);
+  if (directoryPath == null) {
+    return;
+  }
+  final Directory directory = Directory(directoryPath);
+  final File file = File('${directory.path}${Platform.pathSeparator}$name');
+
+  if (file.existsSync()) {
+    file.deleteSync();
+    expect(file.existsSync(), isFalse);
+  }
+
+  file.writeAsStringSync('Hello world!');
+  expect(file.readAsStringSync(), 'Hello world!');
+  expect(directory.listSync(), isNotEmpty);
+  file.deleteSync();
+}