Merge branch 'foundation' into yczhang

Runtime/foundation/change_notifier.cs

         }
     }
 
-    class _MergingListenable : ChangeNotifier {
+    class _MergingListenable : Listenable {
-
-            foreach (Listenable child in _children) {
-                if (child != null) {
-                    child.addListener(this.notifyListeners);
-                }
-            }
-        public override void dispose() {
+        public void addListener(VoidCallback listener) {
-                if (child != null) {
-                    child.removeListener(this.notifyListeners);
-                }
+                child?.addListener(listener);
+        }
-            base.dispose();
+        public void removeListener(VoidCallback listener) {
+            foreach (Listenable child in this._children) {
+                child?.removeListener(listener);
+            }
         }
 
         public override string ToString() {

Runtime/foundation/constants.cs

+using UnityEngine;
+
+namespace Unity.UIWidgets.foundation {
+    public class FoundationConstants {
+        public bool kReleaseMode = !Debug.isDebugBuild;
+    }
+}

Runtime/foundation/constants.cs.meta

+fileFormatVersion: 2
+guid: 36d19e17f3d947f98c7accec2cd5771b
+timeCreated: 1556097117