isIgnored({source}: {+source: ?string, ...}): boolean {
    return source != null && this._getIgnoredSourceSet().has(source);
  }
Returns `true` if the given source is in this map's ignore list, `false`
otherwise.

When used with the `source-map` package, you'll first use
`SourceMapConsumer#originalPositionFor` to retrieve a source location,
then pass that location to `isIgnored`.