LearnTS Mixins uses super.method() in solution without it being previously introduced

[apxamccallum]

I was unable to complete this lesson as there was no instruction on how to reference items from the source object. Maybe super should be included with the inheritance lesson?

interactive-tutorials/tutorials/learn-ts.org/en/Mixins.md

Lines 95 to 101 in febfd6a

	activate() {
	super.activate();
	this.log("Activating...");
	}
	deactivate() {
	super.deactivate();
	this.log("Deactivating...");