Index Pattern Not Preserved In Dashboards Export

[evanfreed]

Kibana version:
6.4.1

Elasticsearch version:
6.4.1

Server OS version:
OSX (local)

Browser version:
Chrome

Original install method (e.g. download page, yum, from source, etc.):
Download page

Describe the bug:
This is an extension of what I mentioned in #22541 (comment) where exporting dashboards and reimporting them was not setting a default index pattern. However, it looks like the export is not finding the pattern anymore which is likely causing the reimport issues (see below).

When using the Rest API to pull down dashboards to save into a file I'm seeing the index-pattern section as having a 404 not found (in previous versions, at least in 6.2.4, this was not the case).

...
{
      "id": "metricbeat-*",
      "type": "index-pattern",
      "error": {
        "statusCode": 404,
        "message": "Not found"
	}
}
...

Steps to reproduce:

1. Setup ES, Kibana, Metricbeat locally
2. Import Metricbeat dashboard (metricbeat-6.4.1-darwin-x86_64/kibana/6/dashboard)

curl -XPOST -H 'kbn-xsrf:true' -H 'Content-type:application/json' localhost:5601/api/kibana/dashboards/import -d @Metricbeat-host-overview.json

3. Export to file (populate ENTER_DASHBOARD_ID with correct ID)

curl -XGET -H 'kbn-xsrf:true' -H 'Content-type:application/json' localhost:5601/api/kibana/dashboards/export?dashboard=ENTER_DASHBOARD_ID > Metricbeat-host-overview-version-2.json

4. Inspect File for index-pattern.

Expected behavior:
In previous versions when pulling the dashboard you would get associated index-patterns as well. It seems like this behavior has changed or there was a regression of some kind.

I'm not entirely sure if this API makes any additional calls to get the index-patterns but from what I noticed locally, as well as what's mentioned in #20242 (comment), it appears that the api/saved_objects/_find API has suffered a regression

curl -XGET "localhost:9200/api/saved_objects/_find?type=index-pattern&search_fields=title&search=metric*"
{"error":{"root_cause":[{"type":"illegal_argument_exception","reason":"request [/api/saved_objects/_find] contains unrecognized parameters: [search], [search_fields] -> did you mean [stored_fields]?"}],"type":"illegal_argument_exception","reason":"request [/api/saved_objects/_find] contains unrecognized parameters: [search], [search_fields] -> did you mean [stored_fields]?"},"status":400

This seems like a second issue but I wanted to bring it up in case the export dashboards API leverages it or similar logic.

@bmcconaghy

[ThomThomson]

Can confirm that this is no longer an issue, with our extract and inject references logic and the excellent export saved objects API.