get lastKnownByteLength() {
    if (this._rawSrc) {
      return this._rawSrc.length;
    } else if (typeof this._lastKnownByteLength === 'number') {
      return this._lastKnownByteLength;
    } else {
      return this.rawSrc.length; // Force the rawSrc to be computed
    }
  }
Get the last known byt length of the Asset

Doesn't force a serialization of the asset if a value has previously been recorded.

@type {Number}