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

TBR=abarth

Review URL: https://codereview.chromium.org/1234093002 .

sdk/lib/rendering/auto_layout.dart

@@ -102,7 +102,8 @@ class AutoLayoutParentData extends BoxParentData
     // 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