Change _coordinate to internal.

Author: Yuncong Zhang

Runtime/widgets/scroll_position.cs

@@ -47,7 +47,7 @@ protected ScrollPosition(
 
         public readonly string debugLabel;
 
-        public readonly object _coordinator;
+        internal readonly object _coordinator;
 
         public float minScrollExtent {
             get { return this._minScrollExtent.Value; }