Images fail to load without a string literal containing its full name

For example, the program below will fail with `Image 'creatures/OhNoes' was not found` until a string literal containing "OhNoes" in its entirety appears anywhere in the program.

```
image(getImage('creatures/Oh' + 'Noes'), 0, 20);

for(var i = 0; i < 3; i++){
var avatar = 'creatures/Hopper-' + ['Cool', 'Happy', 'Jumping'][i];
image(getImage(avatar), i * 140, 200);
}

// var a = "creatures/" + 'OhNoes';
// var b = {"Hopper-Cool": 0};
// var c = '#####Hopper-HappyHopper-Jumping#####';
```

Try uncommenting the last three lines one by one.

Start by reproducing the example in the issue and trace the image/getImage asset lookup in the live editor. Verify the behavior with the concatenated names shown, then confirm that images load without a full-name string literal and that existing image loading still works.

Written by the indexing model from the issue text.