walk(aFn) {
    let chunk
    for (let i = 0, len = this.children.length; i < len; i++) {
      chunk = this.children[i]
      if (chunk[isSourceNode]) {
        chunk.walk(aFn)
      } else if (chunk !== '') {
        aFn(chunk, {
          source: this.source,
          line: this.line,
          column: this.column,
          name: this.name,
        })
      }
    }
  }
Walk over the tree of JS snippets in this node and its children. The
walking function is called once for each snippet of JS and is passed that
snippet and the its original associated source's line/column location.

@param aFn The traversal function.