Use Fuchsia system compositor for physical model layers. (#11164)

On Fuchsia, the system compositor is responsible for drawing shadows
for physical model layers with non-zero elevation.

Also fixed a bug where _needsCompositing was not being cleared prior
to updating it.

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

@@ -2113,6 +2113,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     if (!_needsCompositingBitsUpdate)
       return;
     final bool oldNeedsCompositing = _needsCompositing;
+    _needsCompositing = false;
     visitChildren((RenderObject child) {
       child._updateCompositingBits();
       if (child.needsCompositing)

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

@@ -1286,6 +1286,7 @@ class RenderPhysicalModel extends _RenderCustomClip<RRect> {
     if (elevation == value)
       return;
     _elevation = value;
+    markNeedsCompositingBitsUpdate();
     markNeedsPaint();
   }
 
@@ -1325,6 +1326,11 @@ class RenderPhysicalModel extends _RenderCustomClip<RRect> {
   static final Paint _defaultPaint = new Paint();
   static final Paint _transparentPaint = new Paint()..color = const Color(0x00000000);
 
+  // On Fuchsia, the system compositor is responsible for drawing shadows
+  // for physical model layers with non-zero elevation.
+  @override
+  bool get alwaysNeedsCompositing => _elevation != 0.0 && defaultTargetPlatform == TargetPlatform.fuchsia;
+
   @override
   void paint(PaintingContext context, Offset offset) {
     if (child != null) {

packages/flutter/test/rendering/proxy_box_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:test/test.dart';
 
@@ -29,4 +30,43 @@ void main() {
     layout(makeFittedBox(), constraints: new BoxConstraints.tight(Size.zero), phase: EnginePhase.paint);
     expect(painted, equals(false));
   });
+
+  test('RenderPhysicalModel compositing on Fuchsia', () {
+    debugDefaultTargetPlatformOverride = TargetPlatform.fuchsia;
+
+    final root = new RenderPhysicalModel(color: new Color(0xffff00ff));
+    layout(root, phase: EnginePhase.composite);
+    expect(root.needsCompositing, isFalse);
+
+    // On Fuchsia, the system compositor is responsible for drawing shadows
+    // for physical model layers with non-zero elevation.
+    root.elevation = 1.0;
+    pumpFrame(phase: EnginePhase.composite);
+    expect(root.needsCompositing, isTrue);
+
+    root.elevation = 0.0;
+    pumpFrame(phase: EnginePhase.composite);
+    expect(root.needsCompositing, isFalse);
+
+    debugDefaultTargetPlatformOverride = null;
+  });
+
+  test('RenderPhysicalModel compositing on non-Fuchsia', () {
+    debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
+
+    final root = new RenderPhysicalModel(color: new Color(0xffff00ff));
+    layout(root, phase: EnginePhase.composite);
+    expect(root.needsCompositing, isFalse);
+
+    // On non-Fuchsia platforms, Flutter draws its own shadows.
+    root.elevation = 1.0;
+    pumpFrame(phase: EnginePhase.composite);
+    expect(root.needsCompositing, isFalse);
+
+    root.elevation = 0.0;
+    pumpFrame(phase: EnginePhase.composite);
+    expect(root.needsCompositing, isFalse);
+
+    debugDefaultTargetPlatformOverride = null;
+  });
 }