* update gallery and web samples compatable with SDK version 2a576b774c: Flutter 1.12.3-pre.38 • channel master • git@github.com:flutter/flutter.git Framework • revision 2a576b774c (2 hours ago) • 2019-11-15 13:15:05 -0800 Engine • revision b2640d97e7 Tools • Dart 2.7.0 * add preview image