Migrate off deprecated isInstanceOf (dart-lang/source_span#40)

- Bump `package:test` to `1.6.0` which introduce `isA`.
- Use `2.1.0` SDK on travis since it's the minimum supported by `test`.
- Bump min SDK in the pubspec to `2.1.0` to match what is tested.
- Replace `new isInstanceOf` with `isA`.