[android] Fail if all devices drop offline while running tests.

build/android/pylib/base/test_dispatcher_unittest.py

Index: build/android/pylib/base/test_dispatcher_unittest.py
diff --git a/build/android/pylib/base/test_dispatcher_unittest.py b/build/android/pylib/base/test_dispatcher_unittest.py
index 88dfda69f01939a1253b8b7739a3207cf3dadcc2..c6078dbdfcfca6f0bb433067139f8158cd181920 100644
--- a/build/android/pylib/base/test_dispatcher_unittest.py
+++ b/build/android/pylib/base/test_dispatcher_unittest.py
@@ -186,6 +186,15 @@ class TestShard(unittest.TestCase):
     self.assertEqual(len(results.GetAll()), 0)
     self.assertEqual(exit_code, constants.ERROR_EXIT_CODE)
 
+  def testTestsRemainWithAllDevicesOffline(self):
+    attached_devices = android_commands.GetAttachedDevices
+    android_commands.GetAttachedDevices = lambda: []
+    try:
+      with self.assertRaises(AssertionError):
+        results, exit_code = TestShard._RunShard(MockRunner)
+    finally:
+      android_commands.GetAttachedDevices = attached_devices
+
 
 class TestReplicate(unittest.TestCase):
   """Tests test_dispatcher.RunTests with replication."""