Historically (up to JDK 1.8), documentation for the `java.awt.Toolkit` class
referred to the "awt.toolkit" system property, which was set to the
name of the platform implementation subclass.
The documentation of this property was removed in JDK 9 because
(1) it was an internal detail not intended to be a supported interface
and (2) encapsulation by the module system meant that the class could
not be directly manipulated.
However, the system property was not removed until now.
Applications that previously read this property can still, if
necessary, obtain it by instantiating the AWT Toolkit and querying
the class name via that instance.