setSourceContent(aSourceFile, aSourceContent) {
    let source = aSourceFile
    if (this._sourceRoot != null) {
      source = util.relative(this._sourceRoot, source)
    }

    if (aSourceContent != null) {
      // Add the source content to the _sourcesContents map.
      // Create a new _sourcesContents map if the property is null.
      if (!this._sourcesContents) {
        this._sourcesContents = Object.create(null)
      }
      this._sourcesContents[util.toSetString(source)] = aSourceContent
    } else if (this._sourcesContents) {
      // Remove the source file from the _sourcesContents map.
      // If the _sourcesContents map is empty, set the property to null.
      delete this._sourcesContents[util.toSetString(source)]
      if (Object.keys(this._sourcesContents).length === 0) {
        this._sourcesContents = null
      }
    }
  }
Set the source content for a source file.