A support tech from ESRi told us about this beta web app, designed to enable JSON editing capability of ArcGIS Online web apps and maps:
We would love to see ESRI continue development of this tool and begin formal support! Our initial tests have been very positive, particularly with our dev - QA - production workflow of ArcGIS Online web apps. This tool makes it really fast and easy to save and reuse the info window popup configurations, as one example. Please continue the good work on this!