I'm afraid the link by Aslan is not useful. They just say that the statement is true, but - like any other site with that quote - they don't give a reference or any other proof that it is really from Anne Frank. Obviously there is no other solution than going through the whole book. But perhaps this is just what Rupert has done.
A suspicious feature is the paronomasis (similarity) defy : define, which exists in English (and to a lesser extent in French), but not in Dutch (the original language of the Diary).
Strangely I found the statement only once in Dutch on the web: “Kijk hoe een kaars de duisternis zowel kan trotseren als definiëren.” Either the Dutch hate to quote this on their web pages, or the statement has really been coined in English.