1. A method comprising: in response to receiving an initial user input to build an application package for an application: assembling, by a builder computing system, at least a portion of source code of the application and a set of resources of the application into the application package for the application, wherein the set of resources of the application includes one or more of: a build script, source code that defines an appearance of a graphical user interface of the application, markup language that defines the appearance of the application, a text file, or an image; and deploying, by the builder computing system, the application package to a test environment system to initiate execution of the application within the test environment system, wherein the application executes as a plurality of threads in the test environment system; and while the application is executing in the test environment system and after modification to at least a subset of the set of resources: receiving, at the builder computing system, a subsequent user input that causes the builder computing system to assemble an updated subset of the set of resources using the subset of the set of resources that were modified; and deploying, by the builder computing system and to the test environment system, the updated subset of the set of resources to update the application without sending another application package that includes the updated subset of the set of resources, wherein the updated subset of the set of resources is deployed to a temporary filesystem location of the test environment system, wherein deploying the updated subset of the set of resources causes a coordinator component of the test environment system to restart a subset of plurality of threads of the application to cause the application to execute with the updated subset of resources without restarting the application.