add a basic integration test for Flutter

scripts/slave/recipes/flutter/flutter.py

Index: scripts/slave/recipes/flutter/flutter.py
diff --git a/scripts/slave/recipes/flutter/flutter.py b/scripts/slave/recipes/flutter/flutter.py
index 7f7e34eadd0b20eb25b5f27e6bad0b8893ea791a..ca3dfb9c2aa110fee57c1e71aa8fce918a667be5 100644
--- a/scripts/slave/recipes/flutter/flutter.py
+++ b/scripts/slave/recipes/flutter/flutter.py
@@ -54,6 +54,12 @@ def TestFlutterPackagesAndExamples(api):
 
   _flutter_test('examples/stocks')
 
+def TestCreateAndLaunch(api):
+  with MakeTempDir(api) as temp_dir:
+    api.step('test create', ['flutter', 'create', '--with-driver-test',
+        'sample_app'], cwd=temp_dir)
+    app_path = temp_dir.join('sample_app')
+    api.step('drive sample_app', ['flutter', 'drive'], cwd=app_path)
 
 # TODO(eseidel): Would be nice to have this on api.path or api.file.
 @contextlib.contextmanager
@@ -157,6 +163,11 @@ def RunSteps(api):
     TestFlutterPackagesAndExamples(api)
     BuildExamples(api, git_hash)
 
+    # TODO(yjbanov): we do not yet have Android devices hooked up, nor do we
+    # support the Android emulator. For now, only run on iOS Simulator.
+    if api.platform.is_mac:
+      TestCreateAndLaunch(api)
+
     # TODO(eseidel): We only want to generate one copy of the docs at a time
     # otherwise multiple rsyncs could race, causing badness. We'll eventually
     # need both a lock on the bucket, as well as some assurance that we're