Update flutter_tools to use package:file throughout (#7385)

This removes direct file access from within flutter_tools
in favor of using `package:file` via a `FileSystem` that's
accessed via the `ApplicationContext`.

This lays the groundwork for us to be able to easily swap
out the underlying file system when running Flutter tools,
which will be used to provide a record/replay file system,
analogous to what we have for process invocations.

packages/flutter_tools/lib/src/ios/xcodeproj.dart

diff --git a/packages/flutter_tools/lib/src/ios/xcodeproj.dart b/packages/flutter_tools/lib/src/ios/xcodeproj.dart
index 6d1bc49..3d440e1 100644
--- a/packages/flutter_tools/lib/src/ios/xcodeproj.dart
+++ b/packages/flutter_tools/lib/src/ios/xcodeproj.dart
@@ -2,10 +2,9 @@
 // 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:path/path.dart' as path;
 
+import '../base/file_system.dart';
 import '../base/process.dart';
 import '../build_info.dart';
 import '../cache.dart';
@@ -23,7 +22,7 @@
   localsBuffer.writeln('FLUTTER_ROOT=$flutterRoot');
 
   // This holds because requiresProjectRoot is true for this command
-  String applicationRoot = path.normalize(Directory.current.path);
+  String applicationRoot = path.normalize(fs.currentDirectory.path);
   localsBuffer.writeln('FLUTTER_APPLICATION_PATH=$applicationRoot');
 
   // Relative to FLUTTER_APPLICATION_PATH, which is [Directory.current].
@@ -43,7 +42,7 @@
   if (tools.isLocalEngine)
     localsBuffer.writeln('LOCAL_ENGINE=${tools.engineBuildPath}');
 
-  File localsFile = new File(path.join(projectPath, 'ios', 'Flutter', 'Generated.xcconfig'));
+  File localsFile = fs.file(path.join(projectPath, 'ios', 'Flutter', 'Generated.xcconfig'));
   localsFile.createSync(recursive: true);
   localsFile.writeAsStringSync(localsBuffer.toString());
 }