getImportableSymbolReferencesForDocument is not a function
I've seen this work in the samples and in other plug-ins, but it fails in my instance.

```
var sketch = require('sketch');
var Library = require('sketch/dom').Library;
var document = sketch.getSelectedDocument(); //there is a doc open, and it does have linked libraries
var symbolReferences=Library.getImportableSymbolReferencesForDocument(document);
```

The failure reported to console is 

> TypeError: Library.getImportableSymbolReferencesForDocument is not a function. (In 'Library.getImportableSymbolReferencesForDocument(document)', 'Library.getImportableSymbolReferencesForDocument' is undefined)

I'm using Sketch 54.1