constructor(consumer, path) {
    super(consumer);

    var url = consumer.sourceRoot ? join(consumer.sourceRoot, path) : path;
    consumer.eachMapping((mapping) => {
      if (url && mapping.source !== url)
        return;
      var line = mapping.originalLine;
      this.columnsByLine_[line] = this.columnsByLine_[line] || [];
      this.columnsByLine_[line].push(mapping.originalColumn);
    }, this, SourceMapConsumer.ORIGINAL_ORDER);
  }
@param {SourceMapConsumer} consumer
@param {string} path which source to analyze