Add support for backdrop filters

For example, to implement backdrop blur effects.

bin/cache/engine.version

-b8151a8d9bb2fd2c1d67e8007a53eba5489f3d0d
+13f67343d008c9673f528ec2451c7d5a0528605c

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

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:ui' as ui show Picture, SceneBuilder;
+import 'dart:ui' as ui show ImageFilter, Picture, SceneBuilder;
 import 'dart:ui' show Offset;
 
 import 'package:flutter/painting.dart';
@@ -448,3 +448,18 @@ class ShaderMaskLayer extends ContainerLayer {
     description.add('transferMode: $transferMode');
   }
 }
+
+/// A composited layer that applies a filter to the existing contents of the scene.
+class BackdropFilterLayer extends ContainerLayer {
+  BackdropFilterLayer({ this.filter });
+
+  /// The filter to apply to the existing contents of the scene.
+  ui.ImageFilter filter;
+
+  @override
+  void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
+    builder.pushBackdropFilter(filter);
+    addChildrenToScene(builder, layerOffset);
+    builder.pop();
+  }
+}

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

@@ -3,7 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:developer';
-import 'dart:ui' as ui show PictureRecorder;
+import 'dart:ui' as ui show ImageFilter, PictureRecorder;
 
 import 'package:flutter/gestures.dart';
 import 'package:flutter/painting.dart';
@@ -333,6 +333,19 @@ class PaintingContext {
     painter(childContext, offset);
     childContext._stopRecordingIfNeeded();
   }
+
+  /// Push a backdrop filter.
+  ///
+  /// This function applies a filter to the existing painted content and then
+  /// synchronously calls the painter to paint on top of the filtered backdrop.
+  void pushBackdropFilter(Offset offset, ui.ImageFilter filter, PaintingContextCallback painter) {
+    _stopRecordingIfNeeded();
+    BackdropFilterLayer backdropFilterLayer = new BackdropFilterLayer(filter: filter);
+    _appendLayer(backdropFilterLayer);
+    PaintingContext childContext = new PaintingContext._(backdropFilterLayer, _paintBounds);
+    painter(childContext, offset);
+    childContext._stopRecordingIfNeeded();
+  }
 }
 
 /// An encapsulation of a renderer and a paint() method.

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

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:ui' as ui show ImageFilter;
+
 import 'package:flutter/gestures.dart';
 import 'package:vector_math/vector_math_64.dart';
 
@@ -673,6 +675,34 @@ class RenderShaderMask extends RenderProxyBox {
   }
 }
 
+class RenderBackdropFilter extends RenderProxyBox {
+  RenderBackdropFilter({ RenderBox child, ui.ImageFilter filter })
+    : _filter = filter, super(child) {
+    assert(filter != null);
+  }
+
+  ui.ImageFilter get filter => _filter;
+  ui.ImageFilter _filter;
+  void set filter (ui.ImageFilter newFilter) {
+    assert(newFilter != null);
+    if (_filter == newFilter)
+      return;
+    _filter = newFilter;
+    markNeedsPaint();
+  }
+
+  @override
+  bool get alwaysNeedsCompositing => child != null;
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    if (child != null) {
+      assert(needsCompositing);
+      context.pushBackdropFilter(offset, _filter, super.paint);
+    }
+  }
+}
+
 /// A class that provides custom clips.
 abstract class CustomClipper<T> {
   /// Returns a description of the clip given that the render object being

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

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:ui' as ui show Image;
+import 'dart:ui' as ui show Image, ImageFilter;
 
 import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
@@ -122,6 +122,28 @@ class ShaderMask extends SingleChildRenderObjectWidget {
   }
 }
 
+class BackdropFilter extends SingleChildRenderObjectWidget {
+  BackdropFilter({
+    Key key,
+    this.filter,
+    Widget child
+  }) : super(key: key, child: child) {
+    assert(filter != null);
+  }
+
+  final ui.ImageFilter filter;
+
+  @override
+  RenderBackdropFilter createRenderObject(BuildContext context) {
+    return new RenderBackdropFilter(filter: filter);
+  }
+
+  @override
+  void updateRenderObject(BuildContext context, RenderBackdropFilter renderObject) {
+    renderObject.filter = filter;
+  }
+}
+
 /// Paints a [Decoration] either before or after its child paints.
 /// Container insets its child by the widths of the borders, this Widget does not.
 ///