Fix MessageEvent.data to convert using structured clone.

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 45057f413572e1ed721207c56fd8746e8f23ecc8..c306d5b8a1ed9a659a3e21e934ad435565447f64 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -26122,8 +26122,12 @@ class MessageEvent extends Event {
 
   @DomName('MessageEvent.data')
   @DocsEditable()
-  Object get data => wrap_jso(_blink.BlinkMessageEvent.instance.data_Getter_(unwrap_jso(this)));
-  
+  // TODO(alanknight): This really should be generated by the
+  // _OutputConversion in the systemnative.py script, but that doesn't
+  // use those conversions right now, so do this as a one-off.
+  dynamic get data => convertNativeToDart_SerializedScriptValue(
+      _blink.BlinkMessageEvent.instance.data_Getter_(unwrap_jso(this)));
+
   @DomName('MessageEvent.lastEventId')
   @DocsEditable()
   @Unstable()