Explicitly dispose heavy C++ objects

This patch calls dispose() explicitly on a number of heavy C++ objects so that
they can be eagerly finalized instead of waiting for GC.

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

@@ -59,6 +59,10 @@ abstract class Layer {
     _nextSibling = null;
     _previousSibling = null;
     _parent = null;
+    _dispose();
+  }
+
+  void _dispose() {
   }
 
   /// Override this function to upload this layer to the engine
@@ -84,6 +88,11 @@ class PictureLayer extends Layer {
   /// The picture's coodinate system matches this layer's coodinate system
   ui.Picture picture;
 
+  void _dispose() {
+    super._dispose();
+    picture.dispose();
+  }
+
   void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
     builder.addPicture(offset + layerOffset, picture, paintBounds);
   }
@@ -181,6 +190,7 @@ class ContainerLayer extends Layer {
     child._previousSibling = null;
     child._nextSibling = null;
     child._parent = null;
+    child._dispose();
   }
 
   /// Removes all of this layer's children from its child list
@@ -191,12 +201,23 @@ class ContainerLayer extends Layer {
       child._previousSibling = null;
       child._nextSibling = null;
       child._parent = null;
+      child._dispose();
       child = next;
     }
     _firstChild = null;
     _lastChild = null;
   }
 
+  void _dispose() {
+    super._dispose();
+    Layer child = _firstChild;
+    while (child != null) {
+      Layer next = child.nextSibling;
+      child._dispose();
+      child = next;
+    }
+  }
+
   void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
     addChildrenToScene(builder, offset + layerOffset);
   }

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

@@ -122,7 +122,9 @@ class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox>
       Rect bounds = Point.origin & (size * ui.window.devicePixelRatio);
       ui.SceneBuilder builder = new ui.SceneBuilder(bounds);
       layer.addToScene(builder, Offset.zero);
-      ui.window.render(builder.build());
+      ui.Scene scene = builder.build();
+      ui.window.render(scene);
+      scene.dispose();
     } finally {
       ui.tracing.end('RenderView.compositeFrame');
     }