1. A computer-implemented method performed by data processing apparatus comprising one or more computers in data communication, the method comprising: determining, for a native application that generates an application environment for display on a user device within the native application, the native application operating independent of a browser application that can operate on the user device, a set of environment instances of the native application, the determining comprising receiving a set of uniform resource identifiers for the native application from a publisher of the native application, each uniform resource identifier in the set of uniform resource identifiers corresponding to a corresponding environment instance in the set of environment instances; for each environment instance: determining textual data describing features of the corresponding environment instance, the textual data being data that is not rendered to be visible when the native application renders the environment instance on a user device display, the determining of textual data comprising: instantiating a virtual machine emulating an operating system of a user device; instantiating, within the virtual machine, the native application; accessing, within the virtual machine, the uniform resource identifier to which the environment instance corresponds, and in response to accessing each uniform resource identifier: generating in the virtual machine the environment instance that corresponds to the uniform resource identifier; and extracting the textual data provided to a rendering process of the native application, wherein the textual data is identified as invisible textual data for the rendering process so that the textual data is not rendered to be visible when the native application renders the environment instance on a user device display; generating, from the textual data, native application environment instance data describing content of the environment instance; and indexing the native application environment instance data for the native application in an index that is searchable by a search engine.