wrong error when using a unit struct as length of a repeat expression

``` rust
struct A { i: i32};
let a = [0; A{i: 42}];
```

yields the expected

```
error: expected positive integer for repeat count, found struct [E0306]
```

while 

``` rust
struct B;
let b = [0; B];
```

thinks that `B` is a variable

```
error: expected constant integer for repeat count, found variable [E0307]
```

[PlayPen](http://is.gd/IgyXUb)
