code: func (f *uploadBytesFuture) Get() (blob.Ref, error) {
  	for _, f := range f.children {
  		if _, err := f.Get(); err != nil {
  			return blob.Ref{}, err
  		}
  	}
  	return f.br, <-f.errc
  }
docs: // Get blocks for all children and returns any final error.