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you can update the dart Sdk easily by Chocolately app
dawnload choco in orginal website
then you can using cmd update the installed dart sdk to newest by this command
choco update dart
The current Dart SDK version is 2.18.6.
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Open the project pubspec yaml file and next to environment: sdk:, change the version number to the minimum version required by the package or app as instructed in the error message.
I had to downgrade my sdk version from from ">=2.17.0-182.0.dev <3.0.0" to ">=2.16.0-182.0.dev <3.0.0".