Updates to debugDisableShadows (#17577)

1. Make CupertinoSwitch use BoxShadow.toPaint() so that it respects
   the `debugDisableShadows` flag.
2. Increase blue radius on debug banner
3. Only stroke "synthetic shadow" borders if elevation is positive.

bin/internal/goldens.version

@@ -1 +1 @@
-df3f713cc47fc4e919d1c5cf52c0543424c26c09
+298b585e6eb2bf51b12cd0ea9261a1b7a83c9a29

packages/flutter/lib/src/cupertino/thumb_painter.dart

@@ -6,8 +6,6 @@ import 'package:flutter/painting.dart';
 
 import 'colors.dart';
 
-final MaskFilter _kShadowMaskFilter = new MaskFilter.blur(BlurStyle.normal, BoxShadow.convertRadiusToSigma(1.0));
-
 /// Paints an iOS-style slider thumb.
 ///
 /// Used by [CupertinoSwitch] and [CupertinoSlider].
@@ -16,7 +14,10 @@ class CupertinoThumbPainter {
   CupertinoThumbPainter({
     this.color: CupertinoColors.white,
     this.shadowColor: const Color(0x2C000000),
-  });
+  }) : _shadowPaint = new BoxShadow(
+         color: shadowColor,
+         blurRadius: 1.0,
+       ).toPaint();
 
   /// The color of the interior of the thumb.
   final Color color;
@@ -24,6 +25,9 @@ class CupertinoThumbPainter {
   /// The color of the shadow case by the thumb.
   final Color shadowColor;
 
+  /// The paint used to draw the shadow case by the thumb.
+  final Paint _shadowPaint;
+
   /// Half the default diameter of the thumb.
   static const double radius = 14.0;
 
@@ -35,17 +39,13 @@ class CupertinoThumbPainter {
   /// Consider using [radius] and [extension] when deciding how large a
   /// rectangle to use for the thumb.
   void paint(Canvas canvas, Rect rect) {
-    final RRect rrect = new RRect.fromRectAndRadius(rect, new Radius.circular(rect.shortestSide / 2.0));
+    final RRect rrect = new RRect.fromRectAndRadius(
+      rect,
+      new Radius.circular(rect.shortestSide / 2.0),
+    );
 
-    final Paint paint = new Paint()
-      ..color = shadowColor
-      ..maskFilter = _kShadowMaskFilter;
-    canvas.drawRRect(rrect, paint);
-    canvas.drawRRect(rrect.shift(const Offset(0.0, 3.0)), paint);
-
-    paint
-      ..color = color
-      ..maskFilter = null;
-    canvas.drawRRect(rrect, paint);
+    canvas.drawRRect(rrect, _shadowPaint);
+    canvas.drawRRect(rrect.shift(const Offset(0.0, 3.0)), _shadowPaint);
+    canvas.drawRRect(rrect, new Paint()..color = color);
   }
 }

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

@@ -1600,13 +1600,15 @@ class RenderPhysicalModel extends _RenderPhysicalModelBase<RRect> {
       bool paintShadows = true;
       assert(() {
         if (debugDisableShadows) {
-          context.canvas.drawRRect(
-            offsetRRect,
-            new Paint()
-              ..color = shadowColor
-              ..style = PaintingStyle.stroke
-              ..strokeWidth = elevation * 2.0,
-          );
+          if (elevation > 0.0) {
+            context.canvas.drawRRect(
+              offsetRRect,
+              new Paint()
+                ..color = shadowColor
+                ..style = PaintingStyle.stroke
+                ..strokeWidth = elevation * 2.0,
+            );
+          }
           paintShadows = false;
         }
         return true;
@@ -1721,13 +1723,15 @@ class RenderPhysicalShape extends _RenderPhysicalModelBase<Path> {
       bool paintShadows = true;
       assert(() {
         if (debugDisableShadows) {
-          context.canvas.drawPath(
-            offsetPath,
-            new Paint()
-              ..color = shadowColor
-              ..style = PaintingStyle.stroke
-              ..strokeWidth = elevation * 2.0,
-          );
+          if (elevation > 0.0) {
+            context.canvas.drawPath(
+              offsetPath,
+              new Paint()
+                ..color = shadowColor
+                ..style = PaintingStyle.stroke
+                ..strokeWidth = elevation * 2.0,
+            );
+          }
           paintShadows = false;
         }
         return true;

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

@@ -110,7 +110,7 @@ class BannerPainter extends CustomPainter {
 
   static const BoxShadow _shadow = const BoxShadow(
     color: const Color(0x7F000000),
-    blurRadius: 4.0,
+    blurRadius: 6.0,
   );
 
   bool _prepared = false;