Include a pull request reference.

pkgs/yaml/test/yaml_test.dart

diff --git a/pkgs/yaml/test/yaml_test.dart b/pkgs/yaml/test/yaml_test.dart
index 053f6be..8da4b8a 100644
--- a/pkgs/yaml/test/yaml_test.dart
+++ b/pkgs/yaml/test/yaml_test.dart
@@ -30,8 +30,8 @@
   });
 
   group("refuses", () {
+    // Regression test for #19.
     test("invalid contents", () {
-      // This caused the scanner to hang in an infinite loop.
       expectYamlFails("{");
     });