Extracted build_tree for a new class.

Signed off by: Alessandro Palmeira <alessandro.palmeira@gmail.com>
Signed off by: Daniel Bucher <daniel.bucher88@gmail.com>
Added tree builder spec.

Signed-off-by: Alessandro Palmeira <alessandro.palmeira@gmail.com>
Signed-off-by: Daniel Alves <danpaulalves@gmail.com>
Removing collect from runner.

Signed-off-by: Alessandro Palmeira <alessandro.palmeira@gmail.com>
Signed-off-by: Daniel Alves <danpaulalves@gmail.com>