1. A computer readable storage medium storing computer-executable instructions that, when executed by a computer system, cause the computer system to perform a method comprising: selecting a first execution runtime in the computer system for execution of a runtime agnostic representation of user code, the first execution runtime selected based on at least one heuristic of the runtime agnostic representation; and invoking a scheduler to dispatch a set of tasks from the runtime agnostic representation for execution on the computer system using the first execution runtime.