1. A computer system for, upon request of a requestor of a host application, downloading custom code for execution within the host application, the host application for execution within a host process and the custom code for execution as managed code with the host process, the computer system comprising: a memory storing computer-executable instructions of: a unmanaged custom code loader that, when requested by the requestor, starts execution of a managed code runtime environment and invokes managed code of the managed code runtime environment to download the custom code; a managed custom code loader that executes within the managed code runtime environment and when invoked by the unmanaged custom code loader component, downloads a manifest for the custom code that specifies components of the custom code and includes information describing how the custom code is to be used; reports to the requestor progress during the download of the manifest so that additional processing can be performed by the requestor when downloading the manifest; downloads the custom code in accordance with the downloaded manifest; verifies trust of the custom code by invoking functionality provided by the requestor; reports to the requestor progress during the download of the custom code so that additional processing can be performed by the requestor when downloading the custom code; and launches execution of the custom code as managed code executing within the managed code runtime environment within the host process; and a component that uninstalls the custom code when requested by the requestor; and a processor that executes the computer-executable instructions stored in the memory.