Importing legacy json file throws index pattern exception

[flash1293]

Kibana version: master

Describe the bug: Importing a legacy json file exported from 6.8 sems to successfully import all objects, but an exception is thrown and the user sees a technical warning about not being able to clear the index pattern cache

Steps to reproduce:

1. Go to management > saved objects
2. Open import flyout
3. Drop this file: https://gist.githubusercontent.com/flash1293/0ea158f94dd165647c560d1b90a5ed72/raw/e322a98549a4893589d36e1f42a431b66113a979/export.json
4. Start import
5. Error is shown: this.patternCache.clear is not a function

Expected behavior:
Success message shown

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[pgayvallet]

Seems like the relevant call is here, in IndexPattern.save:

kibana/src/plugins/data/public/index_patterns/index_patterns/index_pattern.ts

Lines 462 to 463 in 911981d

	// Clear cache
	this.patternCache.clear(this.id);

@elastic/kibana-app-arch I think this is an issue in the data plugin, not in the SOM consumer. Would you mind taking a look?

[ppisljar]

taking a look