1. A computer readable storage medium, which does not include transitory propagating signals, storing computer-executable instructions that, when executed by a computer system, cause the computer system to perform a method comprising: selecting, solely by one and only one runtime-agnostic library, a first execution runtime in the computer system for execution of a runtime agnostic representation of user code including representing inherent parallelism in the user code, the first execution runtime selected based on at least one heuristic of the runtime agnostic representation and compute resources of the computer system available to the first execution runtime; and invoking, solely by the one and only one runtime-agnostic library, a scheduler to dispatch a set of tasks from the runtime agnostic representation for execution on the computer system using the first execution runtime.