emsgSchemeIdUris_(inBandEventStreams, emsgSchemeIdUris) {
    const schemeIdUris = emsgSchemeIdUris.slice();
    for (const event of inBandEventStreams) {
      const schemeIdUri = event.attributes['schemeIdUri'];
      if (!schemeIdUris.includes(schemeIdUri)) {
        schemeIdUris.push(schemeIdUri);
      }
    }
    return schemeIdUris;
  }
Returns a new array of InbandEventStream schemeIdUri containing the union
of the ones parsed from inBandEventStreams and the ones provided in
emsgSchemeIdUris.

@param {!Array<!shaka.extern.xml.Node>} inBandEventStreams
    Array of InbandEventStream
    elements to parse and add to the returned array.
@param {!Array<string>} emsgSchemeIdUris Array of parsed
    InbandEventStream schemeIdUri attributes to add to the returned array.
@return {!Array<string>} schemeIdUris Array of parsed
    InbandEventStream schemeIdUri attributes.
@private