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.
In the body below this function, why is it not

```TypeScript
if (needCollisionCheckForIdentifier(node, name, "_this")) {
    potentialThisCollisions.push(node);
}
```