Stop using final fields in JSObject

It is not allowed by the API docs and now also forbidden in practice

Author: patrick96

web-image/src/com.oracle.svm.webimage.jtt/src/com/oracle/svm/webimage/jtt/api/JSObjectSubclassTest.java

@@ -162,6 +162,7 @@ private static void heapGeneratedObjects() {
         inspectHeapOnlyPoint("dummy");
     }
 
+    // TODO GR-65036 This will have to be updated since the x and y values will be JS primitives
     @JS("const { x, y } = dummy.$vm.exports.com.oracle.svm.webimage.jtt.api.HeapOnlyPointStatics.getPoint(); console.log(x.$as('number')); console.log(y.$as('number'));")
     private static native void inspectHeapOnlyPoint(String dummy);
 }
@@ -212,7 +213,7 @@ public String toString() {
 
 @JS.Export
 class SubclassOfImportedJSObject extends ImportedJSObject {
-    protected final int index;
+    protected int index;
 
     SubclassOfImportedJSObject(String importDeclaration, int index) {
         super(importDeclaration);