The v2.8 update includes the new capabilities that were part of the Web AppBuilder for ArcGIS online release this month as well as new documentation specifically on unit tests. See the ArcGIS blog below by David Martinez.
Starting with Web AppBuilder for ArcGIS (Developer Edition) 2.8, the beta version for each release will be available in Early Adopter Community right after Web AppBuilder in AGOL update. So you can test the latest features and enhancements in Developer Edition beta as well as import the apps from AGOL when needed. Currently, Developer Edition 2.8 beta syncs up with the latest Web AppBuilder in AGOL April 11th update. Click on this link to access the 2.8 beta release.
Developer Edition 2.8 has more widgets for 2D apps, including Data Aggregation (beta),Distance and Direction, Gridded Reference Graphic, and Emergency Response Guide. For 3D apps, the dashboard theme and Measurement and Share widgets are available now. Key enhancements have been made to Edit for related feature editing, Legend for choosing layers to show legends, Smart Editor for editing related layers and tables as well as for defining attribute actions, and the statistic layer from the extra data source for a new option of the filter by extent. See What’s Newfrom AGOL update.More SDK related topics will be added in the final release.
Please use the Forums within the Early Adopter Community to communicate all your observations and feedback.