Unexpected range assignment behaviour

[Lewis]

```
string[3][string] lookup;
string[] dynArray = ["d", "e", "f"];
lookup["test"] = dynArray[0..$];
```

This fails at runtime with RangeError. But if I change that last 
line to:

```
lookup["test"] = dynArray[0..3];
```

then it works. But the value of $ here is 3. Why do I get a 
RangeError at runtime even though the slice is the correct size 
(and the same size as the hardcoded one that works)? I would have 
expected to only get a RangeError if at runtime the value turned 
out to be wrong.