1. A method of loading source files, comprising: receiving, in the course of executing a script, an indication that a source file on which a current class or function depends is not available; continuing execution of remaining portions of the script, if any, that do not depend on the source file, or continuing on to other processing tasks if no such portions remain, while the source file is loaded asynchronously; receiving an indication that the source file has been loaded; and executing a portion of the script that depends on the source file; and wherein the portion of the script that depends on the source file comprises a method that includes programmatically in a definition of the current class or function the methods and variables of a super-class with which the source file is associated.