message: Fix plugin tests for latest nightly.
diff: @@ -375,7 +375,7 @@ fn panic_abort_plugins() {
  "bar/src/lib.rs",
  r#"
  #![feature(rustc_private)]
  - extern crate syntax;
  + extern crate rustc_ast;
  "#,
  )
  .build();
  @@ -427,7 +427,7 @@ fn shared_panic_abort_plugins() {
  "bar/src/lib.rs",
  r#"
  #![feature(rustc_private)]
  - extern crate syntax;
  + extern crate rustc_ast;
  extern crate baz;
  "#,
  )
  