Unsafe method calls permitted incorrectly

This may be fallout from recent borrow checker changes. Some examples:

Yields an ICE:

```
[06:53:54] <dbaupp> rusti: unsafe { (~[]).unsafe_get(1) } --> ICE!

Both of these work, one warns that the unsafe block is not needed:
```

[06:54:28] <dbaupp> rusti: unsafe { (~[1i,2]).unsafe_get(1) }
[06:55:08] <nmatsakis> rusti: (~[1i,2]).unsafe_get(1)
[06:55:09] <rusti> 2

```
```
