Improved memory usage in Espruino

Just tested latest build(http://www.espruino.com/binaries/git/commits/8e7aac987b82d5843face571789fdab1e2c6cc5b/espruino_1v70_espruino_1r3.bin) with this:

var  l = false,led = LED1;
setInterval(function() {
  l = !l;
  led.write(l);
}, 1500);

And got Uncaught Error: Field or method does not exist.....
May be an "older bug". I did not test for some time.