Assets don't work for Dash apps created with an existing Flask server
This is because the `name` param is taken to be the default value from the constructor (namely `__main__`), even if a Flask instance is provided. In this case, the `name` used to locate the base path of the package should be updated to be the `name` attribute of the Flask instance.
diff --git a/dash/dash.py b/dash/dash.py
index fb14dbee..4c78af58 100644
--- a/dash/dash.py
+++ b/dash/dash.py
@@ -96,6 +96,7 @@ class Dash(object):
                 See https://github.com/plotly/dash/issues/141 for details.
                 ''', DeprecationWarning)
 
+        name = name if server is None else server.name
         self._assets_folder = assets_folder or os.path.join(
             flask.helpers.get_root_path(name), 'assets'
         )
['dash/dash.py']