[Gallery] Fix directory structure (#312)

gallery/tool/codeviewer_cli/main.dart

@@ -0,0 +1,31 @@
+// Copyright 2019 The Flutter team. 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:io';
+
+import 'package:args/args.dart';
+import 'package:path/path.dart' as path;
+
+import 'segment_generator.dart';
+
+void main(List<String> arguments) {
+  final parser = ArgParser()
+    ..addOption(
+      'target',
+      help: 'The file path for the output target file.',
+      defaultsTo: path.join(
+          Directory.current.path, 'lib', 'codeviewer', 'code_segments.dart'),
+    )
+    ..addFlag(
+      'dry-run',
+      help: 'Write the output to stdout.',
+    );
+  final argResults = parser.parse(arguments);
+
+  writeSegments(
+    sourceDirectoryPath: path.join(Directory.current.path, 'lib', 'demos'),
+    targetFilePath: argResults['target'] as String,
+    isDryRun: argResults['dry-run'] as bool,
+  );
+}