Compatible with facebook HHVM 3.1

This patch to make HTML5 compatible with HHVM contains also workaround to avoid this https://github.com/facebook/hhvm/issues/2962
Of course I could take care of this in PHP_CodeCoverage. But IMHO it should be taken care of in HHVM. HHVM's code coverage API should not expose data for `systemlib*`.

@ptarjan @sgolemon