Handle breakage after libcore split

API Changes:

- &[T] and ~[T] no longer support the addition operator (+)

src/libcore/lib.rs

@@ -111,4 +111,6 @@ mod std {
     #[cfg(test)] pub use realstd::rt;     // needed for fail!()
     #[cfg(test)] pub use realstd::option; // needed for assert!()
     #[cfg(test)] pub use realstd::os;     // needed for tests
+    #[cfg(test)] pub use realstd::slice;  // needed for tests
+    #[cfg(test)] pub use realstd::vec;    // needed for vec![]
 }