1. A method of loading source files, comprising: determining, in the course of executing a script, that a current class or function of the script depends on a source file that is not available, wherein the determination is made when the current class is called or the function is executed during the execution of the script; in response to determining, initiating an asynchronous load of the source file; and while the source file is asynchronously loading: if the script includes remaining portions that do not depend on the source file, then continue executing the remaining portions of the script, otherwise carry out other processing tasks.