Fix local import paths. (#14183)

This is in attempt to resolve build failure on Windows.

packages/flutter_tools/lib/src/test/flutter_platform.dart

@@ -5,8 +5,6 @@
 import 'dart:async';
 import 'dart:convert';
 
-import 'package:flutter_tools/src/artifacts.dart';
-import 'package:flutter_tools/src/compile.dart';
 import 'package:meta/meta.dart';
 import 'package:stream_channel/stream_channel.dart';
 
@@ -14,10 +12,12 @@ import 'package:test/src/backend/test_platform.dart'; // ignore: implementation_
 import 'package:test/src/runner/plugin/platform.dart'; // ignore: implementation_imports
 import 'package:test/src/runner/plugin/hack_register_platform.dart' as hack; // ignore: implementation_imports
 
+import '../artifacts.dart';
 import '../base/common.dart';
 import '../base/file_system.dart';
 import '../base/io.dart';
 import '../base/process_manager.dart';
+import '../compile.dart';
 import '../dart/package_map.dart';
 import '../globals.dart';
 import 'watcher.dart';