Capture closures around megamorphic dispatches

The performLayout and build callsite are highly megamorphic because they
dispatch into a large number of clients. However, for a given caller, the
callee is always of the same type, which means the megamorphic lookup exactly
factors by the caller. We can speed up the dispatch by capturing a closure at
initialization and then monomorphically dispatching through the closure.

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

@@ -777,6 +777,9 @@ class PipelineOwner {
 
 }
 
+// See _performLayout.
+void _doNothing() { }
+
 /// An object in the render tree.
 ///
 /// The [RenderObject] class hierarchy is the core of the rendering
@@ -804,6 +807,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
 
   RenderObject() {
     _needsCompositing = isRepaintBoundary || alwaysNeedsCompositing;
+    _performLayout = performLayout;
   }
 
   // LAYOUT
@@ -1073,7 +1077,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       return true;
     });
     try {
-      performLayout();
+      _performLayout();
       markNeedsSemanticsUpdate();
     } catch (e, stack) {
       _debugReportException('performLayout', e, stack);
@@ -1168,7 +1172,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       return true;
     });
     try {
-      performLayout();
+      _performLayout();
       markNeedsSemanticsUpdate();
       assert(() { debugAssertDoesMeetConstraints(); return true; });
     } catch (e, stack) {
@@ -1235,6 +1239,11 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   /// information without informing this render object.
   void performLayout();
 
+  // We cache a closure to performLayout so that the callsite is monomorphic.
+  // Initializing this field with _buildNothing helps the compiler prove that
+  // this field always holds a closure.
+  VoidCallback _performLayout = _doNothing;
+
   /// Allows this render object to mutate its child list during layout and
   /// invokes callback.
   void invokeLayoutCallback(LayoutCallback callback) {

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

@@ -1351,12 +1351,18 @@ abstract class BuildableElement extends Element {
 
 typedef Widget WidgetBuilder(BuildContext context);
 
+// See _builder.
+Widget _buildNothing(BuildContext context) => null;
+
 /// Base class for the instantiation of [StatelessWidget], [StatefulWidget],
 /// and [_ProxyWidget] widgets.
 abstract class ComponentElement extends BuildableElement {
   ComponentElement(Widget widget) : super(widget);
 
-  WidgetBuilder _builder;
+  // Initializing this field with _buildNothing helps the compiler prove that
+  // this field always holds a closure.
+  WidgetBuilder _builder = _buildNothing;
+
   Element _child;
 
   @override
@@ -1456,7 +1462,7 @@ class StatefulElement extends ComponentElement {
     assert(_state._debugTypesAreRight(widget));
     assert(_state._element == null);
     _state._element = this;
-    assert(_builder == null);
+    assert(_builder == _buildNothing);
     _builder = _state.build;
     assert(_state._config == null);
     _state._config = widget;