Merge in the flutter gallery (#176)

gallery/codeviewer_cli/bin/main.dart

@@ -0,0 +1,12 @@
+// 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 'package:codeviewer_cli/segment_generator.dart';
+
+main(List<String> arguments) {
+  writeSegments(
+    sourceDirectoryPath: '../gallery/lib/demos',
+    targetFilePath: '../gallery/lib/codeviewer/code_segments.dart',
+  );
+}