Mark document.createElement as having no side effects.

Relies on changes in ae4e25a77d21031051bfad4391d736712af7b949 to preserve HTML5 SHIM code.

If ae4e25a77d21031051bfad4391d736712af7b949  is rolled back, this commit will need to be as well. See discussion at https://groups.google.com/forum/#%21searchin/closure-compiler-discuss/createElements/closure-compiler-discuss/JzeqEHmEQAQ/vCeT40K1Lm0J