Migrate to stable null-safety.

[CaramelDunes]

Also remove the DoH parts as this was written for Web support that is never going to work as you cannot send UDP packets in a browser. Hopefully fixes #28.
The Web compatibility indicator should be removed from pub.dev accordingly.

I was also thinking we could let the users resolve the address themselves and pass an InternetAddress instead of a hostname.

[oonid]

Hi @CaramelDunes
some people are actually asking to use this library while they build for the web also.
that's why there's DoH implementation.
so removing the web indicator and DoH would set back this library to some while ago.
About the DoH as "the only option", instead of using condition with InternetAddress.lookup, it's the @knezzz decision, so let's wait for the response.

[CaramelDunes]

Unfortunately, DoH is not enough to make this work on the Web. This package actually doesn't work on Web (just tested). NTP is a protocol based on UDP, and thus cannot work in a browser.

[knezzz]

Thanks for PR, I will try to go over it today and test it out, then I'll merge it.
I think removing DoH is good step since it didn't solve any problem but is just creating more problems

[awaisdev5765]

Error: UnimplementedError
at Object.throw_ [as throw] (http://localhost:55438/dart_sdk.js:5348:11)
at bind (http://localhost:55438/packages/universal_io/src/io_impl_js/sync_socket.dart.lib.js:3434:21)
at bind.next ()
at runBody (http://localhost:55438/dart_sdk.js:39211:34)
at Object._async [as async] (http://localhost:55438/dart_sdk.js:39242:7)
at Function.bind (http://localhost:55438/packages/universal_io/src/io_impl_js/sync_socket.dart.lib.js:3431:20)
at getNtpOffset (http://localhost:55438/packages/ntp/ntp.dart.lib.js:128:63)
at getNtpOffset.next ()
at http://localhost:55438/dart_sdk.js:39191:33
at _RootZone.runUnary (http://localhost:55438/dart_sdk.js:39048:58)
at _FutureListener.thenAwait.handleValue (http://localhost:55438/dart_sdk.js:34034:29)
at handleValueCallback (http://localhost:55438/dart_sdk.js:34594:49)
at Function._propagateToListeners (http://localhost:55438/dart_sdk.js:34632:17)
at _Future.new.[_completeWithValue] (http://localhost:55438/dart_sdk.js:34474:23)
at async._AsyncCallbackEntry.new.callback (http://localhost:55438/dart_sdk.js:34497:35)
at Object._microtaskLoop (http://localhost:55438/dart_sdk.js:39335:13)
at _startMicrotaskLoop (http://localhost:55438/dart_sdk.js:39341:13)
at http://localhost:55438/dart_sdk.js:34848:9

NTP.now() throws UnimplementedError Still