12434 delete golden files; switch to flutter/goldens

12435 add dart:html, dart:js, and dart:js_util to the copy of the Dart SDK used for analysis

12443 Force exit felt tool on sigint, sigterm

12446 Add support for JIT release mode

12447 Reflect selection changes in Firefox for text editing

12448 Make kDoNotResizeDimension public so framework can use it directly

12450 Adds support for 5 mouse buttons

12453 Adding Link SemanticsFlag

12454 Add .mskp file to binary format

12470 [web_ui] Check if a pointer is already down for the specific device

12479 Refactoring text_editing.dart