Hi Andrey
1. I did update it when ECMASCRIPT 5.1 when it came out for version 0.4.6 of language-javascript (according to the ChangeLog). The test suite is very comprehensive.
2. I am not familiar with language-ecmascript. The main feature of language-javascript is that it is written using lex/alex, so is pretty fast. That said, the AST itself could be improved, I was learning when I did it.
3. I think a merge could definitely make sense. I know hjsmin is very intimately tied to language-javascript, so a port might be quite tricky. There are also lots of tests on hjsmin.