Merge branch 'material' into 'master'

Material

See merge request upm-packages/ui-widgets/com.unity.uiwidgets!122

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;

Samples/UIWidgetSample/MaterialSample.cs

 using System.Collections.Generic;
+using RSG;
 using Unity.UIWidgets.foundation;
 using Unity.UIWidgets.material;
 using Unity.UIWidgets.painting;
                     this.setState(() => { this._showBottomSheetCallback = this._showBottomSheet; });
                 }
 
-                return null;
+                return new Promise();
             });
         }