Program doesn't reflect changes in global variable named "length"

Reported in ZenDesk:
https://khanacademy.zendesk.com/agent/tickets/92411

When a global variable is named length and the value is changed, the new value isn't reflected.
This would have to do with our injectCode function.

```
var length = 0; //only works with name "length" and a numerical value (stranger if value = "").
draw = function() {
background(0,0,0);
text(length+"\n"+"some text after",10,10,400,400);
};
```

Reproduce the example with a global variable named length, then trace the injectCode function to find where the value is captured or displayed. Verify the fix by changing length and confirming the rendered text updates to the new value.

Written by the indexing model from the issue text.