code: function scanTemplateStart(beginIndex) {
    if (isAtEnd()) {
      reportError('Unterminated template literal', beginIndex, index);
      return lastToken = createToken(END_OF_FILE, beginIndex);
    }
  
    return nextTemplateLiteralTokenShared(NO_SUBSTITUTION_TEMPLATE,
                                          TEMPLATE_HEAD);
  }
docs: Either returns a NO_SUBSTITUTION_TEMPLATE or TEMPLATE_HEAD token.