Unicode character in codeblock causes panic when generating docs

[frewsxcv]

/// ```
/// ⤹
/// ```
pub fn hello() {}

$ rustdoc src/lib.rs
<stdin>:1:1: 1:2 error: unknown start of token: \u{2939}
<stdin>:1 ⤹
          ^
thread '<unnamed>' panicked at 'Box<Any>', ../src/libsyntax/parse/lexer/mod.rs:1437
stack backtrace:
   1:        0x112fe9c58 - sys::backtrace::tracing::imp::write::hd2c80f5e6164b288Sju
   2:        0x112ff21b5 - panicking::default_handler::_$u7b$$u7b$closure$u7d$$u7d$::closure.42908
   3:        0x112ff1d78 - panicking::default_handler::h36a516d1790b538eZDy
   4:        0x112fb9c56 - sys_common::unwind::begin_unwind_inner::h6bef346cfa00530fajt
   5:        0x1126dff4a - sys_common::unwind::begin_unwind::h6729809262858594397
   6:        0x1128396db - parse::lexer::StringReader<'a>::advance_token::h54f1833f3776d092IqQ
   7:        0x1127a14f8 - parse::lexer::StringReader<'a>::new::hdbc77310923600fbKkQ
   8:        0x10e7cd7dc - html::highlight::highlight::h3e69eaed83a4f2afpJk
   9:        0x10e7deb16 - html::markdown::render::block::h544f70e3c42533fcMen
  10:        0x10e85108e - parse_block
  11:        0x10e84fdbb - hoedown_document_render
  12:        0x10e7ddc6c - html::markdown::render::hbb537f99409b54b3wen
  13:        0x11305c2e2 - fmt::write::hdde38ba94d06d154UPW
  14:        0x10e81509a - html::render::Item<'a>.fmt..Display::fmt::h86f1889e147c995a2Mp
  15:        0x11305c270 - fmt::write::hdde38ba94d06d154UPW
  16:        0x10e7fe0a5 - io::Write::write_fmt::h6354941089789537137
  17:        0x10e80bfe8 - html::render::Context::item::render::hff7faac6750ad929Zpp
  18:        0x10e80708b - html::render::Context::item::_$u7b$$u7b$closure$u7d$$u7d$::closure.47094
  19:        0x10e6ac639 - html::render::run::ha2ba16beac6d4602X8n
  20:        0x10e68abf0 - main_args::hfbb54fde3c4db223u5u
  21:        0x10e686c69 - sys_common::unwind::try::try_fn::h10608942960322992620
  22:        0x112fe75ab - __rust_try
  23:        0x112fdfa33 - sys_common::unwind::inner_try::h0d91b1e766cf5ae7cgt
  24:        0x10e6870f2 - boxed::F.FnBox<A>::call_box::h16444054492140399515
  25:        0x112ff11af - sys::thread::Thread::new::thread_start::h964aafa589ae520eqSx
  26:     0x7fff890d2c12 - _pthread_body
  27:     0x7fff890d2b8f - _pthread_start

$ rustc --version
rustc 1.8.0-nightly (57c357d89 2016-02-16)
$ rustdoc --version
rustdoc 1.8.0-nightly (57c357d89 2016-02-16)

servo/webrender#197

[larsbergstrom]

See also: servo/servo#9704

[nox]

#12834