NNBD Migration

[maxlapides]

It's particularly important for this Sentry package to upgrade to Dart NNBD (non-null by default) because this package is incompatible with the null-safe version of the http library. The http library has a breaking change in 0.13.0. So, force-upgrading http to 0.13.0 will result in this error:

../../.pub-cache/hosted/pub.dartlang.org/sentry-4.0.4/lib/src/noop_client.dart:25:20: Error: The method 'NoOpClient.delete' has fewer named arguments than those of overridden method 'Client.delete'.
Future<Response> delete(url, {Map<String, String> headers}) => _response;

Because of this incompatibility, it is not currently possible to upgrade the http package to the null-safe version, which is in turn blocking us from upgrading other packages as well. Thank you!

[maxlapides]

Sorry, duplicate of #247