update 1.17 to Dart 2.8.4 stable

hash: https://github.com/dart-lang/sdk/commit/caebd6700d5ece73b5566b33ff1daecb91dac500

covers:

 - https://github.com/dart-lang/sdk/issues/42111 
 - https://github.com/dart-lang/sdk/issues/42112 