Move surface-based SceneBuilder implementation under surface/

ci/licenses_golden/licenses_flutter

@@ -415,6 +415,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/surface/opacity.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/surface/picture.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/surface/platform_view.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/surface/scene.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/surface/scene_builder.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/surface/surface.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/surface/transform.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/test_embedding.dart

lib/web_ui/lib/src/engine.dart

@@ -82,6 +82,7 @@ part 'engine/surface/opacity.dart';
 part 'engine/surface/picture.dart';
 part 'engine/surface/platform_view.dart';
 part 'engine/surface/scene.dart';
+part 'engine/surface/scene_builder.dart';
 part 'engine/surface/surface.dart';
 part 'engine/surface/transform.dart';
 part 'engine/test_embedding.dart';

lib/web_ui/lib/src/engine/surface/scene.dart

@@ -4,6 +4,27 @@
 
 part of engine;
 
+class SurfaceScene implements ui.Scene {
+  /// This class is created by the engine, and should not be instantiated
+  /// or extended directly.
+  ///
+  /// To create a Scene object, use a [SceneBuilder].
+  SurfaceScene(this.webOnlyRootElement);
+
+  final html.Element webOnlyRootElement;
+
+  /// Creates a raster image representation of the current state of the scene.
+  /// This is a slow operation that is performed on a background thread.
+  Future<ui.Image> toImage(int width, int height) {
+    throw UnsupportedError('toImage is not supported on the Web');
+  }
+
+  /// Releases the resources used by this scene.
+  ///
+  /// After calling this function, the scene is cannot be used further.
+  void dispose() {}
+}
+
 /// A surface that creates a DOM element for whole app.
 class PersistedScene extends PersistedContainerSurface {
   PersistedScene(PersistedScene oldLayer) : super(oldLayer) {