Your equations  only good if dt is very small.
The error will accumulate very quickly.

1. Copy
x = L*Math.sin(q);
y = -L*Math.cos(q);
PE = m*9.8*(L+y);
KE = 0.5*m*L*L*w*w;
to [u][b]Fixed relation[/b][/u] page
2. remove the current [b]evolution[/b] page
3. Add an [b]ODE page[/b]
At the top two text field enter [b]t [/b]and [b]dt[/b]

4 .enter q at the left first column (it will displayed as dq/dt). enter w at the right column
to set up [b]dq/dt=w[/b] , this will tell EJS to calculate new q(t+dt) from q(t) and w
5. enter another relation to get

[b]dw/dt = -g*Math.sin(q)/L;[/b]

6. select [b]Runge-Kutta 4th order[/b] method
7. [b]re-compile[/b] it again and it will give you correct result.