update dart sdk for flutter

This issue is because of the dart version and by updating flutter the dart version won't update, the way for solving this is, first, you need to be on the dev or master channel,

flutter channel master
flutter upgrade

after that run flutter doctor -v as you see your dart is still an old one

in this step, you should update it manually, as the doc says Dart

for Windows user

choco upgrade dart-sdk

for Mac users if you user brew to install dart

brew upgrade dart

and if you don't, you can install it

 brew tap dart-lang/dart
 brew install dart

wish this solves your issue.

You have to upgrade flutter from cli with flutter upgrade command. Then flutter will use latest version of Dart SDK. Flutter does not use your local Dart Sdk.

It might be due to the fact you are on channel stable which is the most secure builds of the four channels to get updated (channels are listed here)

If you are a bit more curious to try out the new features without having to risk bad builds I advocate channel beta:

Every month, we pick the "best" dev build of the previous month or so, and promote it to beta. These builds have been tested with our codelabs.

where the stable channel has this description:

When we believe we have a particularly good build, we promote it to the stable channel. We intend to do this more or less every quarter, but this may vary. We recommend that you use this channel for all production app releases. We may ship hotfixes to the stable channel for high-priority bugs, although our intent is to do this rarely.

You can change the channel to point at beta by running this command in your terminal: flutter channel beta followed by flutter upgrade to ensure you have got the last release for this particular branch.

Let me know how this affect your situation.