Silence analyzer by explicitly casting the parent to the class we know it is.

sky/sdk/lib/rendering/auto_layout.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'package:cassowary/cassowary.dart' as al;
 import 'package:sky/rendering/box.dart';
 import 'package:sky/rendering/object.dart';
 
 /// Hosts the edge parameters and vends useful methods to construct expressions
 /// for constraints. Also sets up and manages implicit constraints and edit
@@ skipping 84 matching lines @@
   AutoLayoutParentData(this._renderBox) {
     _setupLayoutParameters(this);
   }
 
   final RenderBox _renderBox;
 
   void _applyAutolayoutParameterUpdates() {
     // This is called by the parent's layout function
     // to lay our box out.
     assert(_renderBox.parentData == this);
-    assert(_renderBox.parent.debugDoingThisLayout);
+    assert(_renderBox.parent is RenderAutoLayout);
+    assert((_renderBox.parent as RenderAutoLayout).debugDoingThisLayout); // TODO(ianh): Remove cast once the analyzer is cleverer
     BoxConstraints size = new BoxConstraints.tightFor(
       width: _rightEdge.value - _leftEdge.value,
       height: _bottomEdge.value - _topEdge.value
     );
     _renderBox.layout(size);
     position = new Point(_leftEdge.value, _topEdge.value);
   }
 
   List<al.Constraint> _constructImplicitConstraints() {
     return [
@@ skipping 103 matching lines @@
     defaultPaint(canvas, offset);
   }
 
   List<al.Constraint> _constructImplicitConstraints() {
     // Only edits variables are present on layout containers. If, in the future,
     // implicit constraints (for say margins, padding, etc.) need to be added,
     // they must be returned from here.
     return null;
   }
 }