The ToolBuilder is a framework started by Andreas Raab in 2005. It has been included in Squeak since release 3.9 and is actively maintained. Its aim is to achieve cross-UI-framework (MVC, Morphic, Tweak) support for a limited number of widgets which are critical for various Squeak tools, including (but not limited to)
Changeset to fix references to FillInTheBlank for 3.9g-6590, leaves 108 explicit references to the class, all of which are in Morphic, EToys, or the ToolBuilder classes themselves.