fix update assignments of the form `a[0] *= 5;`.
fix update assignments of the form `a[0] *= 5;`.

we were desugaring that to `%getLocal(a)[0] = %getLocal(a).undefined * 5;`  the perils of using
the JS AST as our intermediate form.

toshok/echojs,toshok/echojs,toshok/echojs,toshok/echojs,toshok/echojs,toshok/echojs,toshok/echojs,toshok/echojs