Javascript changes: API changes

Things in https://github.com/babel/notes/issues/30. Related to https://github.com/babel/babel-upgrade/issues/39

Example:
```diff
- file.delete('espowerAssertionVisitor');
+ file.set('espowerAssertionVisitor', null);
```

Start by reading the referenced Babel notes issue 30 and babel-upgrade issue 39, then locate the API usages related to the example change from file.delete to file.set. Confirm the required JavaScript API updates across the tool and verify them with the available project checks; the issue does not name specific files or tests.

Written by the indexing model from the issue text.