in chrome.declarativeContent, there's a type

[
  {
    "namespace": "declarativeContent",
    "types": [
      {
        "id": "ImageDataType",
        "isInstanceOf": "ImageData",
        "additionalProperties": { "type": "any" },
      },

and is later referenced:

      {
        "id": "SetIcon",
        "type": "object",
        "properties": {
          "imageData": {
            "choices": [
              { "$ref": "ImageDataType" },
              {
                "type": "object",
                "additionalProperties": { "type": "any" }
              }
            ],

which is confusing the externs generator (and my brain and probably developers).

I added a few tweaks to check on instance_of in the externs generation, but it wasn't quite working and we probably need to do better $ref handling in general, I'd guess?

Either way, this was motivated by this Github PR:
https://github.com/google/closure-compiler/pull/2527#issuecomment-318757538