Add profiling for building and painting. (#6652)

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

@@ -76,6 +76,13 @@ bool debugPrintMarkNeedsLayoutStacks = false;
 /// Check the intrinsic sizes of each [RenderBox] during layout.
 bool debugCheckIntrinsicSizes = false;
 
+/// Adds [Timeline] events for every RenderObject painted.
+///
+/// For details on how to use [Timeline] events in the Dart Observatory to
+/// optimize your app, see https://fuchsia.googlesource.com/sysui/+/master/docs/performance.md
+bool debugProfilePaintsEnabled = false;
+
+
 /// Returns a list of strings representing the given transform in a format useful for [RenderObject.debugFillDescription].
 List<String> debugDescribeTransform(Matrix4 transform) {
   List<String> matrix = transform.toString().split('\n').map((String s) => '  $s').toList();

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

@@ -95,12 +95,24 @@ class PaintingContext {
   /// into the layer subtree associated with this painting context. Otherwise,
   /// the child will be painted into the current PictureLayer for this context.
   void paintChild(RenderObject child, Offset offset) {
+    assert(() {
+      if (debugProfilePaintsEnabled)
+        Timeline.startSync('${child.runtimeType}');
+      return true;
+    });
+
     if (child.isRepaintBoundary) {
       _stopRecordingIfNeeded();
       _compositeChild(child, offset);
     } else {
       child._paintWithContext(this, offset);
     }
+
+    assert(() {
+      if (debugProfilePaintsEnabled)
+        Timeline.finishSync();
+      return true;
+    });
   }
 
   void _compositeChild(RenderObject child, Offset offset) {

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

@@ -48,6 +48,12 @@ bool debugPrintScheduleBuildForStacks = false;
 /// This can help track down framework bugs relating to the [GlobalKey] logic.
 bool debugPrintGlobalKeyedWidgetLifecycle = false;
 
+/// Adds [Timeline] events for every Widget built.
+///
+/// For details on how to use [Timeline] events in the Dart Observatory to
+/// optimize your app, see https://fuchsia.googlesource.com/sysui/+/master/docs/performance.md
+bool debugProfileBuildsEnabled = false;
+
 Key _firstNonUniqueKey(Iterable<Widget> widgets) {
   Set<Key> keySet = new HashSet<Key>();
   for (Widget widget in widgets) {

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

@@ -2846,6 +2846,12 @@ abstract class ComponentElement extends BuildableElement {
   /// [rebuild] when the element needs updating.
   @override
   void performRebuild() {
+    assert(() {
+      if (debugProfileBuildsEnabled)
+        Timeline.startSync('${widget.runtimeType}');
+      return true;
+    });
+
     assert(_debugSetAllowIgnoredCallsToMarkNeedsBuild(true));
     Widget built;
     try {
@@ -2868,6 +2874,12 @@ abstract class ComponentElement extends BuildableElement {
       built = new ErrorWidget(e);
       _child = updateChild(null, built, slot);
     }
+
+    assert(() {
+      if (debugProfileBuildsEnabled)
+        Timeline.finishSync();
+      return true;
+    });
   }
 
   @override