code: public void onResolve(ResolutionCallback callback) {
      checkState(this.resolveCallback == null, "callback has already been set.");
      checkState(this.value == null, "value is resolved.");
      this.resolveCallback = checkNotNull(callback);
    }
docs: Registers a callback that is invoked when this global is resolved to its actual value.
  
  <p>NOTE: there is no guarantee that this will ever be called.