#環境
macOS Catalina 10.15.4
dart sdk 2.9.2
Homebrew 2.4.14

https://dart.dev/get-dart
を参考にdart sdkをbrewでインストールしている状況。
#解決法
下記を参考にdart sdkのバージョンを2.6.0に下げる。
https://qiita.com/nghryuki/items/7d65d8f55ea65b95310d

以下参考までに自分の実行コマンド。

cd /usr/local/Homebrew/Library/Taps/dart-lang/homebrew-dart

↑個人によって場所が違うかもしれません。dart.rbがあるフォルダを探してください。

git log dart.rb

git checkout <commitID> dart.rb

brew unlink dart

brew install dart

**-Pro:DeepIE3D_Frontend katayamashunsuke$ brew info dart
Warning: Calling 'devel' blocks in formulae is deprecated! Use 'head' blocks or @-versioned formulae instead.
Please report this issue to the dart-lang/dart tap (not Homebrew/brew or Homebrew/core), or even better, submit a PR to fix it:
  /usr/local/Homebrew/Library/Taps/dart-lang/homebrew-dart/dart.rb:32

dart-lang/dart/dart: stable 2.6.0, devel 2.6.0-dev.8.2
The Dart SDK
https://www.dartlang.org/
/usr/local/Cellar/dart/2.6.0 (474 files, 473.9MB) *
  Built from source on 2020-08-31 at 00:19:03
/usr/local/Cellar/dart/2.9.2 (509 files, 523.7MB)
  Built from source on 2020-08-30 at 19:12:42
From: https://github.com/dart-lang/homebrew-dart/blob/HEAD/dart.rb
==> Options
--devel
	Install development version 2.6.0-dev.8.2
==> Caveats
Please note the path to the Dart SDK:
  /usr/local/opt/dart/libexec

brew info dartで上のように2.6.0の横に*がついてれば完了です。