def dependency_graph(self):
        return self._dependency_graph
C{stop()}ping a L{CooperativeTask} whose iterator has encountered an
dataset/ETHPy150Open VisTrails/VisTrails/vistrails/core/packagemanager.py/PackageManager.dependency_graph--crossover with--dataset/ETHPy150Open twisted/twisted/twisted/test/test_cooperator.py/RunStateTests.test_stopErrored