Fix for Phantom
Fix for Phantom

`includes` is valid ES6 but Phantom doesn't seem to like it: the tests
fail with it set. Reverted to the tried and true `indexOf`. All tests are
passing.

— Elliot Jackson <elliot@elliotekj.com>

rust-lang/crates.io,steveklabnik/crates.io,rust-lang/crates.io,steveklabnik/crates.io,rust-lang/crates.io,steveklabnik/crates.io,steveklabnik/crates.io,rust-lang/crates.io