Add missing license headers to `engine_tool`. (#55636) Address post-submit feedback on https://github.com/flutter/engine/pull/55537#pullrequestreview-2346684376.

diff --git a/tools/engine_tool/lib/src/flutter_tool_interop/device.dart b/tools/engine_tool/lib/src/flutter_tool_interop/device.dart
index 990f6bb..9bb0ef7 100644
--- a/tools/engine_tool/lib/src/flutter_tool_interop/device.dart
+++ b/tools/engine_tool/lib/src/flutter_tool_interop/device.dart

@@ -1,3 +1,7 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 import 'dart:convert';
 
 import 'package:meta/meta.dart';

diff --git a/tools/engine_tool/lib/src/flutter_tool_interop/flutter_tool.dart b/tools/engine_tool/lib/src/flutter_tool_interop/flutter_tool.dart
index 2c4084f..3698c9f 100644
--- a/tools/engine_tool/lib/src/flutter_tool_interop/flutter_tool.dart
+++ b/tools/engine_tool/lib/src/flutter_tool_interop/flutter_tool.dart

@@ -1,3 +1,7 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 import 'dart:convert';
 
 import '../environment.dart';

diff --git a/tools/engine_tool/test/src/matchers.dart b/tools/engine_tool/test/src/matchers.dart
index b27d5c7..adee0f9 100644
--- a/tools/engine_tool/test/src/matchers.dart
+++ b/tools/engine_tool/test/src/matchers.dart

@@ -1,3 +1,7 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 import 'package:engine_tool/src/logger.dart';
 import 'package:logging/logging.dart';
 import 'package:test/test.dart';