Add stub podspec

[in_app_purchase]fix version (flutter#3022)

[in_app_purchase] Fix typo on `simulatesAskToBuyInSandBox` (flutter#3016

)

[google_maps_flutter_web] Allow Markers with icon:null (flutter#2998)

[in_app_purchase] Fix typo on `simulatesAskToBuyInSandBox` (flutter#3016

)

[url_launcher_web] Fix a typo in a test name and fix quote consistency (

flutter#3010)

Fix a typo in a test name and fix quote consistency.

[url_launcher_web] Fix a typo in a test name and fix quote consistency (

flutter#3010)

Fix a typo in a test name and fix quote consistency.

[image_picker_for_web] Safari now can pick more video formats (3gp, m…

…p4, possibly others...) (flutter#3007)

[google_maps_flutter] Fix typo in User Interface example (flutter#3011)

[device_info_platform_interface] Typo in doc fixed (flutter#3006)