[flutter_tool] Cache the path context in the flutter tool (#48250)

recomputing the path context in the getter violates flutter repo style guide and adds a small but measurable overhead to all path operations

packages/flutter_tools/lib/src/base/error_handling_file_system.dart

@@ -7,6 +7,7 @@ import 'dart:io' as io show Directory, File, Link;
 
 import 'package:file/file.dart';
 import 'package:meta/meta.dart';
+import 'package:path/path.dart' as p; // ignore: package_path_import
 
 import '../globals.dart' as globals;
 import 'common.dart' show throwToolExit;
@@ -36,6 +37,21 @@ class ErrorHandlingFileSystem extends ForwardingFileSystem {
 
   @override
   File file(dynamic path) => ErrorHandlingFile(delegate, delegate.file(path));
+
+  // Caching the path context here and clearing when the currentDirectory setter
+  // is updated works since the flutter tool restricts usage of dart:io directly
+  // via the forbidden import tests. Otherwise, the path context's current
+  // working directory might get out of sync, leading to unexpected results from
+  // methods like `path.relative`.
+  @override
+  p.Context get path => _cachedPath ??= delegate.path;
+  p.Context _cachedPath;
+
+  @override
+  set currentDirectory(dynamic path) {
+    _cachedPath = null;
+    delegate.currentDirectory = path;
+  }
 }
 
 class ErrorHandlingFile

packages/flutter_tools/test/general.shard/base/error_handling_file_system_test.dart

@@ -6,6 +6,7 @@ import 'package:file/file.dart';
 import 'package:flutter_tools/src/base/error_handling_file_system.dart';
 import 'package:mockito/mockito.dart';
 import 'package:platform/platform.dart';
+import 'package:path/path.dart' as path; // ignore: package_path_import
 
 import '../../src/common.dart';
 import '../../src/context.dart';
@@ -14,6 +15,7 @@ import '../../src/testbed.dart';
 class MockFile extends Mock implements File {}
 class MockFileSystem extends Mock implements FileSystem {}
 class MockPlatform extends Mock implements Platform {}
+class MockPathContext extends Mock implements path.Context {}
 
 void main() {
   group('throws ToolExit on Windows', () {
@@ -32,6 +34,7 @@ void main() {
       when(windowsPlatform.isWindows).thenReturn(true);
       when(windowsPlatform.isLinux).thenReturn(false);
       when(windowsPlatform.isMacOS).thenReturn(false);
+      when(mockFileSystem.path).thenReturn(MockPathContext());
       testbed = Testbed(overrides: <Type, Generator>{
         Platform: () => windowsPlatform,
       });
@@ -96,4 +99,23 @@ void main() {
       expectedMessage: 'The file is being used by another program',
     );
   });
+
+  test('Caches path context correctly', () {
+    final MockFileSystem mockFileSystem = MockFileSystem();
+    final FileSystem fs = ErrorHandlingFileSystem(mockFileSystem);
+
+    expect(identical(fs.path, fs.path), true);
+  });
+
+  test('Clears cache when CWD changes', () {
+    final MockFileSystem mockFileSystem = MockFileSystem();
+    final FileSystem fs = ErrorHandlingFileSystem(mockFileSystem);
+
+    final Object firstPath = fs.path;
+
+    fs.currentDirectory = null;
+    when(mockFileSystem.path).thenReturn(MockPathContext());
+
+    expect(identical(firstPath, fs.path), false);
+  });
 }