add driver failure test (#6776)

dev/bots/test.sh

@@ -22,6 +22,7 @@ flutter analyze --flutter-repo
 # verify that the tests actually return failure on failure and success on success
 (cd dev/automated_tests; ! flutter test test_smoke_test/fail_test.dart > /dev/null)
 (cd dev/automated_tests; flutter test test_smoke_test/pass_test.dart > /dev/null)
+(cd packages/flutter_driver; ! flutter drive --use-existing-app -t test_driver/failure.dart >/dev/null 2>&1)
 
 COVERAGE_FLAG=
 if [ -n "$TRAVIS" ] && [ "$TRAVIS_PULL_REQUEST" == "false" ]; then

packages/flutter_driver/test_driver/failure.dart

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+void main() {
+  // Dummy. Only needed because driver needs an entry point.
+}

packages/flutter_driver/test_driver/failure_test.dart

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+void main() {
+  // Intentionally fail the test. We want to see driver return a non-zero exit
+  // code when this happens.
+  throw 'Failed';
+}