hasContentsOfAllSources() {
    if (!this.sourcesContent) {
      return false
    }
    return (
      this.sourcesContent.length >= this._sources.size() &&
      !this.sourcesContent.some(function (sc) {
        return sc == null
      })
    )
  }
Return true if we have the source content for every source in the source
map, false otherwise.