Fix and test an edge case in findPackageConfigFile. (#163902)

Closes flutter/flutter#163901.

I'm not aware of a case where this causes user crashes, but it could,
and seems easy to fix/test.

Author: matanlurey

packages/flutter_tools/lib/src/dart/package_map.dart

@@ -34,7 +34,7 @@ File? findPackageConfigFile(Directory dir) {
       return candidatePackageConfigFile;
     }
     final Directory parentDir = candidateDir.parent;
-    if (fileSystem.identicalSync(parentDir.path, candidateDir.path)) {
+    if (fileSystem.path.equals(parentDir.path, candidateDir.path)) {
       return null;
     }
     candidateDir = parentDir;

packages/flutter_tools/test/general.shard/dart/package_map_test.dart

@@ -0,0 +1,49 @@
+// Copyright 2014 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 'package:file/file.dart';
+import 'package:file/memory.dart';
+import 'package:flutter_tools/src/dart/package_map.dart';
+
+import '../../src/common.dart';
+
+void main() {
+  group('findPackageConfigFile', () {
+    late FileSystem fileSystem;
+
+    setUp(() {
+      fileSystem = MemoryFileSystem.test();
+    });
+
+    test('should find an immediate .dart_tool/package_config.json', () {
+      fileSystem.file('.dart_tool/package_config.json').createSync(recursive: true);
+      expect(findPackageConfigFile(fileSystem.currentDirectory), isNotNull);
+    });
+
+    test('should find a parent .dart_tool/package_config.json', () {
+      fileSystem.file('.dart_tool/package_config.json').createSync(recursive: true);
+      fileSystem.currentDirectory.childDirectory('child').createSync(recursive: true);
+      expect(findPackageConfigFile(fileSystem.currentDirectory.childDirectory('child')), isNotNull);
+    });
+
+    test('should not find a .dart_tool/package_config.json in an existing directory', () {
+      fileSystem.currentDirectory.childDirectory('child').createSync(recursive: true);
+      expect(findPackageConfigFile(fileSystem.currentDirectory.childDirectory('child')), isNull);
+    });
+
+    // Regression test: https://github.com/flutter/flutter/issues/163901.
+    test('should not find a .dart_tool/package_config.json in a missing directory', () {
+      expect(findPackageConfigFile(fileSystem.currentDirectory.childDirectory('missing')), isNull);
+    });
+
+    // Regression test: https://github.com/flutter/flutter/issues/163901.
+    test('should find a .dart_tool/package_config.json in a parent of a missing directory', () {
+      fileSystem.file('.dart_tool/package_config.json').createSync(recursive: true);
+      expect(
+        findPackageConfigFile(fileSystem.currentDirectory.childDirectory('missing')),
+        isNotNull,
+      );
+    });
+  });
+}