1. A computer-implemented method, comprising: under the control of one or more computer systems that executes instructions, determining a variation point in an executable portion of an application source; causing a first virtual machine instance to execute the executable portion of the application source; halting the first virtual machine instance at a halting point relative to the variation point to produce a halted virtual machine instance; storing a snapshot of the halted virtual machine instance, the snapshot being usable to instantiate a second virtual machine instance to a state that corresponds to a state of the first virtual machine instance at the halting point; and causing, using the snapshot, the second virtual machine instance to continue execution of the executable portion from the state at the halting point.