Nightly feature fails to build on 2018-05-28 nightly

[dtolnay]

$ cargo build --features nightly

   Compiling unicode-xid v0.1.0
   Compiling proc-macro2 v0.4.3
error[E0599]: no function or associated item named `empty` found for type `proc_macro::TokenStream` in the current scope
  --> src/unstable.rs:45:34
   |
45 |             TokenStream::Nightly(proc_macro::TokenStream::empty())
   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ function or associated item not found in `proc_macro::TokenStream`

rust-lang/rust#51073