One current issue with assertions in skulpt is that, while they don't throw assertion errors the condition is still evaluated
Some conditions are slower than others and so this might then cause a performance issue on hot code.

There are a couple of solutions for this that I can see.

1. we could be cleverer about webpack and remove the condition completely in production mode so that these conditions are never evaluated.
2. change the assertion first argument to an arrow function and only check this condition in dev mode

1 would have the biggest win performance wise.

This pr does option 2 and replaces all Sk.asserts.assert condition statement with an arrow function
the advantage is that the assert condition is only evaluated in dev mode and quicker (if enable_asserts) condition is evaluated instead.

So something like

Sk.asserts.assert(this instanceof Sk.builtin.str)

becomes

Sk.asserts.assert(() => this instanceof Sk.builtin.str)

It also means that those using skulpt can turn on and off assertion statements as they like just using the min.js file

They can just do Sk.asserts.ENABLE_ASSERTS = true rather than worry about different build options.
It could potentially become one of the many undocumented configure arguments.