Make layout_bench.dart 19% faster

Previously we would walk from every node in layout to the root to mark the root
as needing paint. Now we leave dirty bits throughout the tree so that we can
cut off the walk as soon as we find a dirty node.

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

@@ -926,13 +926,15 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   /// writes are coalesced, removing redundant computation.
   void markNeedsPaint() {
     assert(!debugDoingPaint);
-    if (!attached) return; // Don't try painting things that aren't in the hierarchy
-    if (_needsPaint) return;
+    if (!attached)
+      return; // Don't try painting things that aren't in the hierarchy
+    if (_needsPaint)
+      return;
+    _needsPaint = true;
     if (hasLayer) {
       // If we always have our own layer, then we can just repaint
       // ourselves without involving any other nodes.
       assert(_layer != null);
-      _needsPaint = true;
       _nodesNeedingPaint.add(this);
       scheduler.ensureVisualUpdate();
     } else if (parent is RenderObject) {
@@ -948,7 +950,6 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       // then we have to paint ourselves, since nobody else can paint
       // us. We don't add ourselves to _nodesNeedingPaint in this
       // case, because the root is always told to paint regardless.
-      _needsPaint = true;
       scheduler.ensureVisualUpdate();
     }
   }