fix mat reference

com.unity.uiwidgets/Runtime/cupertino/text_theme.cs

 using Unity.UIWidgets.foundation;
-using Unity.UIWidgets.material;
+//using Unity.UIWidgets.material;
 using Unity.UIWidgets.service;
 using Unity.UIWidgets.ui;
 using Unity.UIWidgets.widgets;

com.unity.uiwidgets/Runtime/cupertino/theme.cs

 using System.Runtime.CompilerServices;
 using Unity.UIWidgets.foundation;
-using Unity.UIWidgets.material;
+//using Unity.UIWidgets.material;
 using Unity.UIWidgets.service;
 using Unity.UIWidgets.ui;
 using Unity.UIWidgets.widgets;