walkSourceContents(aFn) {
    for (let i = 0, len = this.children.length; i < len; i++) {
      if (this.children[i][isSourceNode]) {
        this.children[i].walkSourceContents(aFn)
      }
    }

    const sources = Object.keys(this.sourceContents)
    for (let i = 0, len = sources.length; i < len; i++) {
      aFn(util.fromSetString(sources[i]), this.sourceContents[sources[i]])
    }
  }
Walk over the tree of SourceNodes. The walking function is called for each
source file content and is passed the filename and source content.

@param aFn The traversal function.