Fix DomCSSStyleSheetExtension to work with dart2wasm. (#122154)

Fix DomCSSStyleSheetExtension to work with dart2wasm.

Author: eyebrowsoffire

packages/flutter/lib/src/services/dom.dart

@@ -336,8 +336,11 @@ class DomCSSStyleSheet extends DomStyleSheet {}
 /// [DomCSSStyleSheet]'s required extension.
 extension DomCSSStyleSheetExtension on DomCSSStyleSheet {
   /// Inserts a rule into this style sheet.
-  int insertRule(String rule, [int? index]) => js_util
-      .callMethod(this, 'insertRule', <Object>[rule, if (index != null) index]);
+  int insertRule(String rule, [int? index]) =>
+    js_util.callMethod<double>(this, 'insertRule', <Object>[
+      rule,
+      if (index != null) index.toDouble()
+    ]).toInt();
 }
 
 /// A list of token.