[flutter_template_images] Add macOS app icon assets (flutter#1930)

Adds the the AppIcon assets in the macOS app template while
simultaneously updating them to meet Apple's icon guidelines as of macOS
Big Sur.

Ref: https://developer.apple.com/design/human-interface-guidelines/macos/icons-and-images/app-icon/

Issue: flutter/flutter#103371

[go_router_builder] Fixes incorrect separator at location path on Win…

…dows. (flutter#1690)

[go_router_builder] Changes the parameter name of the auto-generated …

…go method from buildContext to context (flutter#1635)

[cross_file] Remove unneeded flutter dependency. (flutter#1712)

[pointer_interceptor] fix integration test (flutter#1675)

[go_router_builder] Readme fixes (flutter#1659)

[go_router]Add pkg:go_router_builder (flutter#1551)

Make corresponding changes to pkg:go_router

Fixes flutter/flutter#99127

Co-authored-by: chunhtai <[email protected]>

[go_router]Add pkg:go_router_builder (flutter#1551)

Make corresponding changes to pkg:go_router

Fixes flutter/flutter#99127

Co-authored-by: chunhtai <[email protected]>

[pigeon] added AST validation phase for generators (flutter#1548)

* [pigeon] added AST validation phase for generators

* added negative test

* updated docstring

[pigeon] Minor fixes to NNBD type handling in Dart (flutter#1543)