Fix the transforms for horizontal glows. (#8252)

Also, make all painters have a toString by default.

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

@@ -1777,6 +1777,9 @@ abstract class CustomPainter {
   /// point that should be considered outside the painted image, and null to use
   /// the default behavior.
   bool hitTest(Point position) => null;
+
+  @override
+  String toString() => '$runtimeType#$hashCode';
 }
 
 /// Provides a canvas on which to draw during the paint phase.

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

@@ -450,16 +450,15 @@ class _GlowingOverscrollIndicatorPainter extends CustomPainter {
         break;
       case AxisDirection.left:
         canvas.save();
-        canvas.translate(0.0, size.height);
-        canvas.rotate(-piOver2);
+        canvas.rotate(piOver2);
+        canvas.scale(1.0, -1.0);
         controller.paint(canvas, new Size(size.height, size.width));
         canvas.restore();
         break;
       case AxisDirection.right:
         canvas.save();
-        canvas.translate(size.width, size.height);
-        canvas.rotate(-piOver2);
-        canvas.scale(1.0, -1.0);
+        canvas.translate(size.width, 0.0);
+        canvas.rotate(piOver2);
         controller.paint(canvas, new Size(size.height, size.width));
         canvas.restore();
         break;

packages/flutter/test/widgets/overscroll_indicator_test.dart

@@ -187,11 +187,11 @@ void main() {
     );
     RenderObject painter = tester.renderObject(find.byType(CustomPaint));
     await slowDrag(tester, const Point(200.0, 200.0), const Offset(5.0, 0.0));
-    expect(painter, paints..rotate(angle: -math.PI / 2.0)..circle()..scale(y: -1.0));
+    expect(painter, paints..rotate(angle: math.PI / 2.0)..circle()..saveRestore());
     expect(painter, isNot(paints..circle()..circle()));
     await slowDrag(tester, const Point(200.0, 200.0), const Offset(-5.0, 0.0));
-    expect(painter, paints..rotate(angle: -math.PI / 2.0)..circle()
-                          ..rotate(angle: -math.PI / 2.0)..scale(y: -1.0)..circle());
+    expect(painter, paints..rotate(angle: math.PI / 2.0)..circle()
+                          ..rotate(angle: math.PI / 2.0)..circle());
 
     await tester.pumpUntilNoTransientCallbacks(const Duration(seconds: 1));
     expect(painter, doesNotOverscroll);
@@ -237,7 +237,7 @@ void main() {
     );
     painter = tester.renderObject(find.byType(CustomPaint));
     await slowDrag(tester, const Point(200.0, 200.0), const Offset(5.0, 0.0));
-    expect(painter, paints..scale(y: -1.0)..rotate(angle: -math.PI / 2.0)..circle(color: const Color(0x0A00FF00)));
+    expect(painter, paints..rotate(angle: math.PI / 2.0)..circle(color: const Color(0x0A00FF00)));
     expect(painter, isNot(paints..circle()..circle()));
 
     await tester.pumpUntilNoTransientCallbacks(const Duration(seconds: 1));
@@ -254,7 +254,7 @@ void main() {
     );
     painter = tester.renderObject(find.byType(CustomPaint));
     await slowDrag(tester, const Point(200.0, 200.0), const Offset(5.0, 0.0));
-    expect(painter, paints..rotate(angle: -math.PI / 2.0)..circle(color: const Color(0x0A0000FF))..scale(y: -1.0));
+    expect(painter, paints..rotate(angle: math.PI / 2.0)..circle(color: const Color(0x0A0000FF))..saveRestore());
     expect(painter, isNot(paints..circle()..circle()));
   });
 }