repr: update for removal of const

Author: thestinger

src/libstd/repr.rs

@@ -189,8 +189,7 @@ impl<'self> ReprVisitor<'self> {
         } else if mtbl == 1 {
             // skip, this is ast::m_imm
         } else {
-            assert_eq!(mtbl, 2);
-            self.writer.write("const ".as_bytes());
+            fail!("invalid mutability value");
         }
     }