Use platform appropriate filepaths when constructing asset bundle (#44221)

packages/flutter_tools/lib/src/build_system/targets/assets.dart

@@ -34,7 +34,7 @@ Future<Depfile> copyAssets(Environment environment, Directory outputDirectory) a
     assetBundle.entries.entries.map<Future<void>>((MapEntry<String, DevFSContent> entry) async {
       final PoolResource resource = await pool.request();
       try {
-        final File file = fs.file(fs.path.join(outputDirectory.path, entry.key));
+        final File file = fs.file(outputDirectory.uri.resolve(entry.key));
         outputs.add(file);
         file.parent.createSync(recursive: true);
         final DevFSContent content = entry.value;

packages/flutter_tools/lib/src/bundle.dart

@@ -182,7 +182,7 @@ Future<void> writeBundle(
     assetEntries.entries.map<Future<void>>((MapEntry<String, DevFSContent> entry) async {
       final PoolResource resource = await pool.request();
       try {
-        final File file = fs.file(fs.path.join(bundleDir.path, entry.key));
+        final File file = fs.file(bundleDir.uri.resolve(entry.key));
         file.parent.createSync(recursive: true);
         await file.writeAsBytes(await entry.value.contentsAsBytes());
       } finally {

packages/flutter_tools/test/general.shard/build_system/targets/assets_test.dart

@@ -2,8 +2,6 @@
 // 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_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/build_system/build_system.dart';
 import 'package:flutter_tools/src/build_system/targets/assets.dart';
@@ -47,14 +45,14 @@ flutter:
     expect(fs.file(fs.path.join(environment.buildDir.path, 'flutter_assets', 'AssetManifest.json')).existsSync(), true);
     expect(fs.file(fs.path.join(environment.buildDir.path, 'flutter_assets', 'FontManifest.json')).existsSync(), true);
     expect(fs.file(fs.path.join(environment.buildDir.path, 'flutter_assets', 'LICENSE')).existsSync(), true);
-    // See https://github.com/flutter/flutter/issues/35293
-    expect(fs.file(fs.path.join(environment.buildDir.path, 'flutter_assets', 'assets/foo/bar.png')).existsSync(), true);
+    expect(fs.file(fs.path.join(environment.buildDir.path, 'flutter_assets', 'assets', 'foo', 'bar.png')).existsSync(), true);
   }));
 
   test('Does not leave stale files in build directory', () => testbed.run(() async {
     await buildSystem.build(const CopyAssets(), environment);
+    final File assetFile = fs.file(fs.path.join(environment.buildDir.path, 'flutter_assets', 'assets', 'foo', 'bar.png'));
 
-    expect(fs.file(fs.path.join(environment.buildDir.path, 'flutter_assets', 'assets/foo/bar.png')).existsSync(), true);
+    expect(assetFile.existsSync(), true);
     // Modify manifest to remove asset.
     fs.file('pubspec.yaml')
       ..createSync()
@@ -65,9 +63,8 @@ flutter:
 ''');
     await buildSystem.build(const CopyAssets(), environment);
 
-    // See https://github.com/flutter/flutter/issues/35293
-    expect(fs.file(fs.path.join(environment.buildDir.path, 'flutter_assets', 'assets/foo/bar.png')).existsSync(), false);
-  }), skip: Platform.isWindows); // See https://github.com/google/file.dart/issues/131
+    expect(assetFile.existsSync(), false);
+  }));
 
   test('FlutterPlugins updates required files as needed', () => testbed.run(() async {
     fs.file('pubspec.yaml')