Parsing and emit for computed properties

This is based on the plan in issue #1082. The type checker support, .d.ts generation and language service support (other than navigation items). I plan to work on the type system side next.
It might not have a name at all. Basically the challenge is we are trying to treat a couple of cases exactly the same:
* There is no name
* There is a name that is not computed
* There is a computed name that's a well known symbol