Improve testing for symbol that may reference this

TypeScript Version: 2.7.0-dev.20171115

See comments with text // TODO: GH#20034 (added in #20036).

[mhegazy]

so what is the issue here?

The issue is that some node types are ignored, so we're being overly pessimistic. For example, anything with a getter is assumed to use a this type, just because we haven't handled that syntax kind yet.

[weswigham]

I can see no more references to #20034 in our codebase.