swagger-codegen contains a template-driven engine to generate documentation, API clients and server stubs in different languages by parsing your OpenAPI / Swagger definition.
It's possible to do this with the Online Swagger Editor by importing the YAML and then choose File -> Convert and save as JSON but still not with the online API it seems. :(
per http://stackoverflow.com/questions/34733253/converting-a-swagger-yaml-file-to-json-from-the-command-line/34750203?noredirect=1#comment57254449_34750203