warning fix

Runtime/material/app_bar.cs

                 return this.centerTitle;
             }
 
-            D.assert(themeData.platform != null);
             switch (themeData.platform) {
                 case RuntimePlatform.IPhonePlayer:
                     return this.actions == null || this.actions.Count < 2;

Runtime/material/flexible_space_bar.cs

                 return this.widget.centerTitle;
             }
 
-            D.assert(themeData.platform != null);
             switch (themeData.platform) {
                 case RuntimePlatform.IPhonePlayer:
                     return true;