handle navigation back button events (within-unity solution)

Runtime/engine/UIWidgetsPanel.cs

         protected virtual void Update() {
             this._displayMetrics.Update();
             UIWidgetsMessageManager.ensureUIWidgetsMessageManagerIfNeeded();
+            
+#if UNITY_ANDROID
+            if (Input.GetKeyDown(KeyCode.Escape)) {
+                this._windowAdapter.withBinding(() => {
+                    WidgetsBinding.instance.handlePopRoute();
+                });
+            }
+#endif
 
             if (!this._viewMetricsCallbackRegistered) {
                 this._viewMetricsCallbackRegistered = true;

Runtime/widgets/binding.cs

 using Unity.UIWidgets.foundation;
 using Unity.UIWidgets.rendering;
 using Unity.UIWidgets.ui;
+using UnityEngine;
 
 namespace Unity.UIWidgets.widgets {
     public interface WidgetsBindingObserver {
 
         public bool removeObserver(WidgetsBindingObserver observer) {
             return this._observers.Remove(observer);
+        }
+
+        public void handlePopRoute() {
+            var idx = -1;
+            
+            void _handlePopRouteSub(bool result) {
+                if (!result) {
+                    idx++;
+                    if (idx >= this._observers.Count) {
+                        Application.Quit();
+                        return;
+                    }
+                    this._observers[idx].didPopRoute().Then(_handlePopRouteSub);
+                }
+            }
+            
+            _handlePopRouteSub(false);
         }
 
         public readonly WidgetInspectorService widgetInspectorService;