Hey,
Realm version: v10.0.0-beta9

Seems like there is a serious bug around the lib/browser/rpc.js file, many of the auth methods are expecting an args array param, while they are receiving scalar values, which cause an args.map is not a function error.
For example (lib/browser/rpc.js line 103:

export function _emailPasswordRPC (args) {
  args = args.map(arg => serialize(null, arg))
  const result = sendRequest('_emailPassword', { arguments: args })
  return deserialize(undefined, result)
}

Calling function is lib/browser/credentials.js line 37:

static emailPassword(email, password) {
  return _emailPasswordRPC(email, password);
}

so rpc function should either be updated to ...args or calling function should pass an array.