Xray panics when scrolling too fast while editing README.md

$ XRAY_SRC_PATH=. RUST_BACKTRACE=1 script/xray .
$ thread 'main' panicked at 'index out of bounds: the len is 30 but the index is 30', /Users/travis/build/rust-lang/rust/src/libcore/slice/mod.rs:2049:10
note: Run with `RUST_BACKTRACE=1` for a backtrace.

$ node --version
$ v8.9.3

$ rustc --version
$ rustc 1.30.0-nightly (33b923fd4 2018-08-18)

$ nvm list
$          v8.9.3
$ ->     v10.12.0
$          system
$ default -> node (-> v10.12.0)
$ node -> stable (-> v10.12.0) (default)
$ stable -> 10.12 (-> v10.12.0) (default)
$ iojs -> N/A (default)
$ lts/* -> lts/carbon (-> N/A)
$ lts/argon -> v4.9.1 (-> N/A)
$ lts/boron -> v6.14.4 (-> N/A)
$ lts/carbon -> v8.12.0 (-> N/A)