flutter_localizations(gen_l10n comamnd) doesn't allow a property name to have a dollar sign ($) in any postion.

Steps to Reproduce

1. Execute flutter gen-l10n involved in any *.arb file which has a key that contains a dollar sign ($).
2. Loot at the terminal output.

Expected results: Dollar sign can be a part of the property name, so it should succeed.

Actual results: gen-l10n failed.

Code sample

In app_en.arb file:

{
  "$address": "Address:"
}

Logs

Invalid ARB resource name "$address" in LocalFile: '<ProjectFolder>\lib/l10n\app_en.arb'.
Resources names must be valid Dart method names: they have to be camel case, cannot start with a number or
underscore, and cannot contain non-alphanumeric characters.