Allow updating the rasterizer tracing threshold from Dart

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

@@ -93,18 +93,22 @@ class StatisticsLayer extends Layer {
   StatisticsLayer({
     Offset offset: Offset.zero,
     this.paintBounds,
-    this.optionsMask
+    this.optionsMask,
+    this.rasterizerThreshold
   }) : super(offset: offset);
 
   /// The rectangle in this layer's coodinate system that bounds the recording
   Rect paintBounds;
 
   /// A mask specifying the statistics to display
-  int optionsMask;
+  final int optionsMask;
+
+  final int rasterizerThreshold;
 
   void addToScene(sky.SceneBuilder builder, Offset layerOffset) {
     assert(optionsMask != null);
     builder.addStatistics(optionsMask, paintBounds.shift(layerOffset));
+    builder.setRasterizerTracingThreshold(rasterizerThreshold);
   }
 
 }

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

@@ -132,11 +132,12 @@ class PaintingContext {
     }
   }
 
-  void paintStatistics(int optionsMask, Offset offset, Size size) {
+  void paintStatistics(int optionsMask, int rasterizerThreshold, Offset offset, Size size) {
     StatisticsLayer statsLayer = new StatisticsLayer(
       offset: offset,
       paintBounds: new Rect.fromLTWH(0.0, 0.0, size.width, size.height),
-      optionsMask : optionsMask
+      optionsMask : optionsMask,
+      rasterizerThreshold : rasterizerThreshold
     );
     _containerLayer.append(statsLayer);
   }

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

@@ -7,7 +7,9 @@ import 'package:sky/src/rendering/object.dart';
 
 class StatisticsBox extends RenderBox {
 
-  StatisticsBox({int optionsMask: 0}) : _optionsMask = optionsMask;
+  StatisticsBox({int optionsMask: 0, int rasterizerThreshold: 0})
+    : _optionsMask = optionsMask,
+      _rasterizerThreshold = rasterizerThreshold;
 
   int _optionsMask;
   int get optionsMask => _optionsMask;
@@ -19,6 +21,16 @@ class StatisticsBox extends RenderBox {
     markNeedsPaint();
   }
 
+  int _rasterizerThreshold;
+  int get rasterizerThreshold => _rasterizerThreshold;
+  void set rasterizerThreshold (int threshold) {
+    if  (threshold == _rasterizerThreshold) {
+      return;
+    }
+    _rasterizerThreshold = threshold;
+    markNeedsPaint();
+  }
+
   bool get sizedByParent => true;
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {
@@ -42,6 +54,6 @@ class StatisticsBox extends RenderBox {
   }
 
   void paint(PaintingContext context, Offset offset) {
-    context.paintStatistics(optionsMask, offset, size);
+    context.paintStatistics(optionsMask, rasterizerThreshold, offset, size);
   }
 }

packages/flutter/lib/src/widgets/statistics_overlay.dart

@@ -40,10 +40,10 @@ class StatisticsOverlay extends LeafRenderObjectWidget {
   /// Create a statistics overlay that only displays specific statistics. The
   /// mask is created by shifting 1 by the index of the specific StatisticOption
   /// to enable.
-  StatisticsOverlay({ this.optionsMask, Key key }) : super(key: key);
+  StatisticsOverlay({ this.optionsMask, this.rasterizerThreshold: 0, Key key }) : super(key: key);
 
   /// Create a statistics overaly that displays all available statistics
-  StatisticsOverlay.allEnabled({ Key key }) : super(key: key), optionsMask = (
+  StatisticsOverlay.allEnabled({ Key key, this.rasterizerThreshold: 0 }) : super(key: key), optionsMask = (
     1 << StatisticsOption.displayRasterizerStatistics.index |
     1 << StatisticsOption.visualizeRasterizerStatistics.index |
     1 << StatisticsOption.displayEngineStatistics.index |
@@ -51,10 +51,15 @@ class StatisticsOverlay extends LeafRenderObjectWidget {
   );
 
   final int optionsMask;
+  final int rasterizerThreshold;
 
-  StatisticsBox createRenderObject() => new StatisticsBox(optionsMask: optionsMask);
+  StatisticsBox createRenderObject() => new StatisticsBox(
+    optionsMask: optionsMask,
+    rasterizerThreshold: rasterizerThreshold
+  );
 
   void updateRenderObject(StatisticsBox renderObject, RenderObjectWidget oldWidget) {
     renderObject.optionsMask = optionsMask;
+    renderObject.rasterizerThreshold = rasterizerThreshold;
   }
 }