In the documentation for the editor you have an execute function called viewSource. However the ts definitions don't list it as an option.
https://www.telerik.com/kendo-angular-ui/components/editor/api/EditorComponent/#toc-opendialog
editor.exec('viewSource');
Come to find out, obviously it makes since that its actually the openDialog method instructions and docs should read:
editor.openDialog('viewSource');