Async gap fixes (dart-lang/stack_trace#96)

* Fix async gap handling. (backpublish)

Fix an issue where an async gap at the end of a stack trace would not
get parsed correctly due to the trailing newline being `trim()`'d.

Add tests to cover this case.

* Fixes async gap handling in Trace.parse and Chain.parse (backpublish)

Co-authored-by: Clement Skau <cskau-g@users.noreply.github.com>