* Code looks good. One tiny note: the FakeCredsManagementTool.find_credentials should return a deferred like the real one does, something like:

255	+ def find_credentials(self):
256	+ """Fake find credentials."""
257	+     if FakeCredsManagementTool.raise_error:
258	+         return defer.fail(Exception('Test!'))
259	+     else:
260	+         return defer.succeed(FakeCredsManagementTool.credentials)

* Tests are not passing on windows:

===============================================================================
[ERROR]
Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\twisted\internet\defer.py", line 1020, in
_inlineCallbacks
    result = g.send(result)
  File "E:\windows-installer\review_fix-reactor-import\ubuntuone_installer\gui\q
t\main\tests\test_windows.py", line 99, in test_failure
    yield windows.check_credentials(
exceptions.AttributeError: 'module' object has no attribute 'check_credentials'

ubuntuone_installer.gui.qt.main.tests.test_windows.CredsFailureMainTestCase.test
_failure
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\twisted\internet\defer.py", line 1020, in
_inlineCallbacks
    result = g.send(result)
  File "E:\windows-installer\review_fix-reactor-import\ubuntuone_installer\gui\q
t\main\tests\test_windows.py", line 118, in test_no_creds
    yield windows.check_credentials(
exceptions.AttributeError: 'module' object has no attribute 'check_credentials'

ubuntuone_installer.gui.qt.main.tests.test_windows.CredsFailureMainTestCase.test
_no_creds
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\twisted\internet\defer.py", line 1020, in
_inlineCallbacks
    result = g.send(result)
  File "E:\windows-installer\review_fix-reactor-import\ubuntuone_installer\gui\q
t\main\tests\test_windows.py", line 136, in test_with_creds
    yield windows.check_credentials(
exceptions.AttributeError: 'module' object has no attribute 'check_credentials'

ubuntuone_installer.gui.qt.main.tests.test_windows.CredsFailureMainTestCase.test
_with_creds
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\twisted\internet\defer.py", line 1020, in
_inlineCallbacks
    result = g.send(result)
  File "E:\windows-installer\review_fix-reactor-import\ubuntuone_installer\gui\q
t\main\tests\test_windows.py", line 163, in test_with_installed_flag
    yield windows.check_credentials(
exceptions.AttributeError: 'module' object has no attribute 'check_credentials'

ubuntuone_installer.gui.qt.main.tests.test_windows.CredsFailureMainTestCase.test
_with_installed_flag
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\twisted\internet\defer.py", line 1020, in
_inlineCallbacks
    result = g.send(result)
  File "E:\windows-installer\review_fix-reactor-import\ubuntuone_installer\gui\q
t\main\tests\test_windows.py", line 152, in test_without_installed_flag
    yield windows.check_credentials(
exceptions.AttributeError: 'module' object has no attribute 'check_credentials'

ubuntuone_installer.gui.qt.main.tests.test_windows.CredsFailureMainTestCase.test
_without_installed_flag