1. A method comprising: obtaining, at one or more data processing apparatus, a package of information defining an extension to an application program interface of a runtime, the package of information including native program code and descriptors that identify different computing platforms supported by the extension; obtaining, at the data processing apparatus, a declaration of a target computing platform for an application program that employs the extension; comparing, at the data processing apparatus, the declaration of the target computing platform for the application program with the descriptors that identify the different computing platforms supported by the extension; if the target computing platform is supported by the extension, generating, at the data processing apparatus, an installable package for the application program; and if the target computing platform is not supported by the extension, providing a notification corresponding thereto.