add(aChunk) {
    if (Array.isArray(aChunk)) {
      aChunk.forEach(function (chunk) {
        this.add(chunk)
      }, this)
    } else if (aChunk[isSourceNode] || typeof aChunk === 'string') {
      if (aChunk) {
        this.children.push(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 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.