src/language-js/parser-babel.js/isFlowFile
function isFlowFile(text, options) {
  if (options.filepath && options.filepath.endsWith(".js.flow")) {
    return true;
  }

  const shebang = getShebang(text);
  if (shebang) {
    text = text.slice(shebang.length);
  }

  const firstNonSpaceNonCommentCharacterIndex = getNextNonSpaceNonCommentCharacterIndexWithStartIndex(
    text,
    0
  );

  if (firstNonSpaceNonCommentCharacterIndex !== false) {
    text = text.slice(0, firstNonSpaceNonCommentCharacterIndex);
  }

  return FLOW_PRAGMA_REGEX.test(text);
}