Add HR regression to benchmark.

pkgs/markdown/benchmark/input.md

diff --git a/pkgs/markdown/benchmark/input.md b/pkgs/markdown/benchmark/input.md
index 31f5f57..1af6a7e 100644
--- a/pkgs/markdown/benchmark/input.md
+++ b/pkgs/markdown/benchmark/input.md
@@ -1,10 +1,16 @@
 **TODO: Add more examples to cover all of the syntax.**
 
+# Regressions
+
+Bad backtracking in the HR parser:
+
+-------------------------- | -------------------------------------------------
+
+# Real-world sample
+
 This input was taken from the test package's README to get a representative
 sample of real-world markdown:
 
-## Writing Tests
-
 Tests are specified using the top-level [`test()`][test] function, and test
 assertions are made using [`expect()`][expect]:
 

pkgs/markdown/benchmark/output.html

diff --git a/pkgs/markdown/benchmark/output.html b/pkgs/markdown/benchmark/output.html
index 6c49aae..71e5771 100644
--- a/pkgs/markdown/benchmark/output.html
+++ b/pkgs/markdown/benchmark/output.html
@@ -1,7 +1,10 @@
 <p><strong>TODO: Add more examples to cover all of the syntax.</strong></p>
+<h1>Regressions</h1>
+<p>Bad backtracking in the HR parser:</p>
+<p>-------------------------- | -------------------------------------------------</p>
+<h1>Real-world sample</h1>
 <p>This input was taken from the test package's README to get a representative
 sample of real-world markdown:</p>
-<h2>Writing Tests</h2>
 <p>Tests are specified using the top-level <a href="http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_test"><code>test()</code></a> function, and test
 assertions are made using <a href="http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_expect"><code>expect()</code></a>:</p>
 <pre class="dart"><code>import "package:test/test.dart";