4138 [in_app_purchase] Fix crash when retrieveReceiptWithError gives an error. (cla: yes, waiting for tree to go green, p: in_app_purchase, platform-ios)

4142 [flutter_plugin_tools] Work around banner in drive-examples (cla: yes, waiting for tree to go green)

4143 [plugin_platform_interface] Fix README broken link (cla: yes, waiting for tree to go green, p: plugin_platform_interface)

4144 [in_app_purchase] Add iOS currency symbol to ProductDetails (cla: yes, waiting for tree to go green, p: in_app_purchase, platform-ios)

4147 Fix webview_flutter Android integration tests and add Espresso (cla: yes, waiting for tree to go green, p: webview_flutter, platform-android)