bzr trunk integration failure with Launchpad tests

[Diogo Matsubara]

As seen in https://pastebin.canonical.com/13807/, Launchpad tests (r7743) are failing to pass due to a integration problem using bzr trunk (r4000) and bzr-loom trunk (r93)

Tests with errors:
   test_simple (canonical.codehosting.puller.tests.test_worker.TestWorkerProgressReporting)
   canonical.codehosting.puller.tests.test_scheduler.TestPullerMasterIntegration.test_mirror canonical.codehosting.puller.tests.test_scheduler.TestPullerMasterIntegration.test_mirror_with_destination_self_locked

Tests with failures:
   canonical.codehosting.puller.tests.test_acceptance.TestBranchPuller.test_hosted_branch_stacked_on_mirrored_branch
   canonical.codehosting.puller.tests.test_acceptance.TestBranchPuller.test_manual_stacking
   canonical.codehosting.puller.tests.test_acceptance.TestBranchPuller.test_mirror_hosted_branch
   canonical.codehosting.puller.tests.test_acceptance.TestBranchPuller.test_mirror_hosted_loom_branch
   canonical.codehosting.puller.tests.test_acceptance.TestBranchPuller.test_mirror_imported_branch
   canonical.codehosting.puller.tests.test_acceptance.TestBranchPuller.test_mirror_mirrored_branch
   canonical.codehosting.puller.tests.test_acceptance.TestBranchPuller.test_mirror_private_branch
   canonical.codehosting.puller.tests.test_acceptance.TestBranchPuller.test_stack_mirrored_branch
   canonical.codehosting.puller.tests.test_acceptance.TestBranchPuller.test_stack_mirrored_branch_onto_private

with the following error:

AssertionError: not equal:
a = None
b = u"AttributeError: 'PullerWorkerUIFactory' object has no attribute '_progress_bar_stack'"