BuiltIn library is created from several helper classes. In version prior to
2.1 they were named Converter, Verify, Variables, RunKeyword and Misc. In
r1512 they were renamed to _Converter, _Verify, _Variables, _RunKeyword and
_Misc to highlight that they are not part of the public API.

Instead of using these individual helper classes, external libraries and
tools should use just he BuiltIn class like "from robot.libraries.BuiltIn
import BuiltIn". This format works with RF 2.1 and also with all earlier
versions. Helper classes are subject to change also in future releases but
the BuiltIn class will stay.