Fix OutlineButton issues.

Runtime/material/button_theme.cs

                 return fillColor;
             }
 
-//todo:xingwei.zhu: uncomment these when OutlineButton are ready
-            if (button is FlatButton /* || button is OutlineButton*/) {
+            if (button is FlatButton || button is OutlineButton) {
                 return null;
             }
 
                         return Colors.white;
                     }
 
-//todo:xingwei.zhu: uncomment these when OutlineButton are ready
-                    if (button is FlatButton /* || button is OutlineButton*/) {
+                    if (button is FlatButton || button is OutlineButton) {
                         return this.colorScheme.primary;
                     }
 
                 return button.splashColor;
             }
 
-//todo:xingwei.zhu: uncomment these when OutlineButton is ready
-            if (this._splashColor != null && (button is RaisedButton /* || button is OutlineButton*/)) {
+            if (this._splashColor != null && (button is RaisedButton || button is OutlineButton)) {
                 return this._splashColor;
             }
 
                 return 0.0f;
             }
 
-//todo:xingwei.zhu: uncomment these when OutlineButton are ready
-//            if (button is OutlineButton)
-//                return 2.0;
+            if (button is OutlineButton)
+                return 2.0f;
             return 8.0f;
         }
 

Runtime/widgets/visibility.cs

             bool maintainInteractivity = false
         ) : base(key: key) {
             D.assert(child != null);
-            D.assert(replacement != null);
             D.assert(maintainState == true || maintainAnimation == false,
                 "Cannot maintain animations if the state is not also maintained.");
             D.assert(maintainAnimation == true || maintainSize == false,