Merge pull request #1297 from Hixie/dialog-PENDING

StackParentData.toString() fix, RenderOpacity default

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -535,7 +535,7 @@ int _getAlphaFromOpacity(double opacity) => (opacity * 255).round();
 /// This class is relatively expensive because it requires painting the child
 /// into an intermediate buffer.
 class RenderOpacity extends RenderProxyBox {
-  RenderOpacity({ RenderBox child, double opacity })
+  RenderOpacity({ RenderBox child, double opacity: 1.0 })
     : _opacity = opacity, _alpha = _getAlphaFromOpacity(opacity), super(child) {
     assert(opacity >= 0.0 && opacity <= 1.0);
   }

packages/flutter/lib/src/rendering/stack.dart

@@ -202,8 +202,9 @@ class StackParentData extends ContainerBoxParentDataMixin<RenderBox> {
       values.add('width=$width');
     if (height != null)
       values.add('height=$height');
-    if (values.length == null)
-      return 'all null';
+    if (values.isEmpty)
+      values.add('not positioned');
+    values.add(super.toString());
     return values.join('; ');
   }
 }