[tool] Update Dart SDK version (#4402)

Since the tool only needs to support back to Flutter 3.3 (the oldest version we still run CI with), this updates the tool to the corresponding minimum Dart version.

This allows the use of `super` parameters, so `dart fix --apply` was run to convert them all (and remove all the includes that were only needed for setting default values).

Also opportunistically cleans up a bunch of unnecessary, very old `dart:async` includes. (Other than those removals, the changes here are all `dart fix`-generated.)

script/tool/lib/src/gradle_check_command.dart

@@ -18,7 +18,7 @@ final Version minKotlinVersion = Version(1, 7, 10);
 /// A command to enforce gradle file conventions and best practices.
 class GradleCheckCommand extends PackageLoopingCommand {
   /// Creates an instance of the gradle check command.
-  GradleCheckCommand(Directory packagesDir) : super(packagesDir);
+  GradleCheckCommand(super.packagesDir);
 
   @override
   final String name = 'gradle-check';