material fix minor

Runtime/material/ink_ripple.cs

                 }
             }
 
-            //todo:xingwei.zhu: remove this condition when drawCircle bug fixed (when radius.value == 0)
-            if (this._radius.value != 0) {
-                canvas.drawCircle(center, this._radius.value, paint);
-            }
-
+            canvas.drawCircle(center, this._radius.value, paint);
             canvas.restore();
         }
     }

Runtime/material/ink_splash.cs

                     canvas.clipRect(rect);
                 }
             }
-
-            //todo:xingwei.zhu: remove this condition when drawCircle bug fixed (when radius.value == 0)
-            if (this._radius.value != 0) {
-                canvas.drawCircle(center, this._radius.value, paint);
-            }
-
+            
+            canvas.drawCircle(center, this._radius.value, paint);
             canvas.restore();
         }
     }

Samples/UIWidgetSample/MaterialSample.cs

         public override Widget build(BuildContext context) {
             return new Material(
                 child: new Center(
-                    child: new MaterialButton(
+                    child: new FlatButton(
+                        shape: new RoundedRectangleBorder(borderRadius: BorderRadius.all(20.0f)),
+                        color: new Color(0xFF00FF00),
+                            child: new Text("Click Me"),
                         onPressed: () => { Debug.Log("pressed here"); }
                     )
                 )