[flutter_tools] ensure desktop builds can run without flutter_tools' package_config (#62426)

packages/flutter_tools/bin/tool_backend.dart

@@ -2,9 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// Do not add package imports to this file.
 import 'dart:convert'; // ignore: dart_convert_import.
 import 'dart:io'; // ignore: dart_io_import.
-import 'package:path/path.dart' as path; // ignore: package_path_import.
 
 /// Executes the required flutter tasks for a desktop build.
 Future<void> main(List<String> arguments) async {
@@ -18,7 +18,7 @@ Future<void> main(List<String> arguments) async {
   final String flutterEngine = Platform.environment['FLUTTER_ENGINE'];
   final String flutterRoot = Platform.environment['FLUTTER_ROOT'];
   final String flutterTarget = Platform.environment['FLUTTER_TARGET']
-    ?? path.join('lib', 'main.dart');
+    ?? pathJoin(<String>['lib', 'main.dart']);
   final String localEngine = Platform.environment['LOCAL_ENGINE'];
   final String projectDirectory = Platform.environment['PROJECT_DIR'];
   final String splitDebugInfo = Platform.environment['SPLIT_DEBUG_INFO'];
@@ -42,9 +42,14 @@ or
 ''');
     exit(1);
   }
-
-  final String flutterExecutable = path.join(
-    flutterRoot, 'bin', Platform.isWindows ? 'flutter.bat' : 'flutter');
+  final String flutterExecutable = pathJoin(<String>[
+    flutterRoot,
+    'bin',
+    if (Platform.isWindows)
+      'flutter.bat'
+    else
+      'flutter'
+  ]);
   final String bundlePlatform = targetPlatform == 'windows-x64' ? 'windows' : 'linux';
   final String target = '${buildMode}_bundle_${bundlePlatform}_assets';
 
@@ -89,3 +94,11 @@ or
     exit(1);
   }
 }
+
+/// Perform a simple path join on the segments based on the current platform.
+///
+/// Does not normalize paths that have repeated separators.
+String pathJoin(List<String> segments) {
+  final String separator = Platform.isWindows ? r'\' : '/';
+  return segments.join(separator);
+}

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

@@ -210,6 +210,16 @@ void main() {
       }
     }
   });
+
+  test('no import of packages in tool_backend.dart', () {
+    final File file = globals.fs.file(globals.fs.path.join(flutterTools, 'bin', 'tool_backend.dart'));
+    for (final String line in file.readAsLinesSync()) {
+      if (line.startsWith(RegExp(r'import.*package:.*'))) {
+        final String relativePath = globals.fs.path.relative(file.path, from:flutterTools);
+        fail('$relativePath imports a package');
+      }
+    }
+  });
 }
 
 bool _isDartFile(FileSystemEntity entity) => entity is File && entity.path.endsWith('.dart');