`dead_code` lint doesn't catch unused `const` values

### STR

``` rust
const FOO: int = 0;
static BAR: int = 0;

fn main() {}
```
### Output

``` rust
const.rs:2:1: 2:21 warning: static item is never used: `BAR`, #[warn(dead_code)] on by default
const.rs:2 static BAR: int = 0;
```
### Version

``` rust
rustc 0.13.0-dev (45797a076 2014-10-10 07:52:00 +0000)
```
