prepend(aChunk) {
    if (Array.isArray(aChunk)) {
      for (let i = aChunk.length - 1; i >= 0; i--) {
        this.prepend(aChunk[i])
      }
    } else if (aChunk[isSourceNode] || typeof aChunk === 'string') {
      this.children.unshift(aChunk)
    } else {
      throw new TypeError(
        'Expected a SourceNode, string, or an array of SourceNodes and strings. Got ' +
          aChunk
      )
    }
    return this
  }
Add a chunk of generated JS to the beginning of this source node.

@param aChunk A string snippet of generated JS code, another instance of
       SourceNode, or an array where each member is one of those things.