The code for the 'Create Spaces from Diagram' tool in the Sketchup Plugin can be found in your OpenStudio installation folder (at least for now), in

/openstudio-2.8.0/SketchUpPlugin/openstudio/sketchup_plugin/lib/dialogs/SpaceDiagramInterface.rb

or here: https://github.com/NREL/OpenStudio/blob/develop/openstudiocore/sketchup_plugin/openstudio/sketchup_plugin/lib/dialogs/SpaceDiagramInterface.rb