Add foreign symbolId
Let's say I have SymbolA in LibraryA. Its Object id is "OID1" and Symbol Id is "SID1". I used this symbol as an override of SymbolB in LibraryB and saved the override value. SymbolA's Symbol ID became "SID2" when imported to LibraryB. So I saved "SID2" as override value. In my DocumentA I need to find and import that symbol which I only know its Symbol Id as "SID2". There is no convenient way to find SymbolA  as an importable for DocumentA. Importable references have only "Future Symbol Id"s. I can get the symbols object id (OID1) but it is useless. The only way to find it by searching with symbol name which may not be unique. Here you can see my example code. 

```js
const getValueForSymbolOverride = (doc, symbol, override) => {
  let master = doc.getSymbolMasterWithID(override.value)
  if (master) {
    return master.symbolId
  } else {
    let library = symbol.master.getLibrary()
    if (library) {
      let importable = library.getImportableSymbolReferencesForDocument(doc)
        .find(importable => importable.id == override.value)
      if (importable) {
        return importable.import().symbolId
      } else {
        master = library.getDocument().getSymbols()
          .find(master => master.symbolId == override.value)
        if (master) {
          if (master.getLibrary()) {
            importable = master.getLibrary()
              .getImportableSymbolReferencesForDocument(doc)
              .find(importable => importable.name == master.name)
          } else {
            importable = library.getImportableSymbolReferencesForDocument(doc)
              .find(importable => importable.name == master.name)
          }
          if (importable) {
            return importable.import().symbolId
          }
        }
      }
    }
  }
}
```

I'm using this code in my [Symbol States](https://github.com/ozgurgunes/Sketch-Symbol-States) plugin. It will be more stable if ImportableObjects have a constant object Id.

Thanks.