_findSourceIndex(aSource) {
    let relativeSource = aSource;
    if (this.sourceRoot != null) {
      relativeSource = util.relative(this.sourceRoot, relativeSource);
    }

    if (this._sources.has(relativeSource)) {
      return this._sources.indexOf(relativeSource)
    }

    // Maybe aSource is an absolute URL as returned by |sources|.  In
    // this case we can't simply undo the transform.
    for (let i = 0; i < this._absoluteSources.length; ++i) {
      if (this._absoluteSources[i] == aSource) {
        return i
      }
    }

    return -1
  }
Utility function to find the index of a source.  Returns -1 if not
found.