Fix array literal/comprehension disambiguation bug

Fix array literal/comprehension disambiguation bug

Parsing "var arrayLit = ['notKeyword', 'for', 'otherStuff'];" will fail because the parser sees a value of 'for' when it gets to the second string literal, and does not verify that it is a Keyword token (and not a StringLiteral).

This fixes it by verifying that the token is not StringLiteral

Perf and coverage runners suggest no regressions

https://code.google.com/p/esprima/issues/detail?id=415