But flutter needs below 2.0.0.

Author: jcollins-g

pubspec.lock

@@ -350,4 +350,4 @@ packages:
     source: hosted
     version: "2.1.13"
 sdks:
-  dart: "2.0.0-dev.6.0"
+  dart: ">=1.23.0 <=2.0.0-dev.6.0"

pubspec.yaml

@@ -5,7 +5,7 @@ author: Dart Team <[email protected]>
 description: A documentation generator for Dart.
 homepage: https://github.com/dart-lang/dartdoc
 environment:
-  sdk: '>=2.0.0-dev.6.0 <3.0.0'
+  sdk: '>=1.23.0-dev.11.5 <2.0.0'
 dependencies:
   analyzer: ^0.30.0
   args: ^0.13.0