Fix static analysis issues for dart 3.6

Author: ntkme

Diff for: lib/src/embedded/js/isolate_main.dart

@@ -10,5 +10,5 @@ import 'js.dart';
 external JSArray<JSAny?> get _argv;
 
 (String, JSArray<JSString>) isolateMain() {
-  return (_argv[1]! as String, _argv.slice(2) as JSArray<JSString>);
+  return ((_argv[1]! as JSString).toDart, _argv.slice(2) as JSArray<JSString>);
 }

Diff for: lib/src/embedded/js/js.dart

@@ -9,7 +9,5 @@ extension JSTypedArrayExtension on JSTypedArray {
 }
 
 extension JSArrayExtension<T extends JSAny?> on JSArray<T> {
-  external int get length;
-  external T operator [](int index);
   external JSArray<T> slice([int start, int end]);
 }

Diff for: pkg/sass_api/pubspec.yaml

@@ -7,7 +7,7 @@ description: Additional APIs for Dart Sass.
 homepage: https://github.com/sass/dart-sass
 
 environment:
-  sdk: ">=3.3.0 <4.0.0"
+  sdk: ">=3.6.0 <4.0.0"
 
 dependencies:
   sass: 1.85.0

Diff for: pubspec.yaml

@@ -8,7 +8,7 @@ executables:
   sass: sass
 
 environment:
-  sdk: ">=3.3.0 <4.0.0"
+  sdk: ">=3.6.0 <4.0.0"
 
 dependencies:
   args: ^2.0.0