code: function getKnownHashesFromAssets(assetMetadata) {
    const knownHashes = new Set();
    for (const metadata of Object.values(assetMetadata)) {
      knownHashes.add(metadata.hash);
    }
    return knownHashes;
  }
docs: Given an assetMetadata mapping, returns a Set of all of the hashes that
  are associated with at least one asset.
  
  @param {Object<string, Object>} assetMetadata Mapping of asset paths to chunk
  name and hash metadata.
  @return {Set} The known hashes associated with an asset.
  
  @private