Bump lower Dart SDK constraints to 3.0 & add class modifiers (#122546)

Bump lower Dart SDK constraints to 3.0 & add class modifiers
diff --git a/dev/tracing_tests/pubspec.yaml b/dev/tracing_tests/pubspec.yaml
index aaf8e88..df98482 100644
--- a/dev/tracing_tests/pubspec.yaml
+++ b/dev/tracing_tests/pubspec.yaml
@@ -2,7 +2,7 @@
 description: Various tests for tracing in flutter/flutter
 
 environment:
-  sdk: '>=2.19.0-0 <4.0.0'
+  sdk: '>=3.0.0-0 <4.0.0'
 
 dependencies:
   flutter: