Introduce a `paints` matcher, and convert the previous customers of (#7567)

MockCanvas to use it instead.

Currently it only supports a very limited set of drawing features, but
adding new ones is easy.

Once the feature set is more complete, and we've used this a bunch, if
it feels right, this API will get promoted to flutter_test.

packages/flutter/test/material/mergeable_material_test.dart

@@ -204,21 +204,12 @@ void main() {
     );
 
     RenderBox box = tester.renderObject(find.byType(MergeableMaterial));
-    MockCanvas canvas = new MockCanvas();
 
-    box.paint(new MockPaintingContext(canvas), Offset.zero);
-
-    final Invocation drawCommand = canvas.invocations.firstWhere((Invocation invocation) {
-      return invocation.memberName == #drawRRect;
-    });
     final BoxShadow boxShadow = kElevationToShadow[2][0];
     final RRect rrect = kMaterialEdges[MaterialType.card].toRRect(
       new Rect.fromLTRB(0.0, 0.0, 800.0, 100.0)
     );
-
-    expect(drawCommand.positionalArguments[0], equals(rrect));
-    expect(drawCommand.positionalArguments[1].color, equals(boxShadow.color));
-    expect(drawCommand.positionalArguments[1].maskFilter, isNotNull);
+    expect(box, paints..rrect(rrect: rrect, color: boxShadow.color, hasMaskFilter: true));
   });
 
   testWidgets('MergeableMaterial merge gap', (WidgetTester tester) async {

packages/flutter/test/material/slider_test.dart

@@ -104,19 +104,11 @@ void main() {
     final RenderBox sliderBox =
         tester.firstRenderObject<RenderBox>(find.byType(Slider));
 
-    Paint getThumbPaint() {
-      final MockCanvas canvas = new MockCanvas();
-      sliderBox.paint(new MockPaintingContext(canvas), Offset.zero);
-      final Invocation drawCommand =
-          canvas.invocations.where((Invocation invocation) {
-        return invocation.memberName == #drawCircle;
-      }).single;
-      return drawCommand.positionalArguments[2];
-    }
-
-    expect(getThumbPaint().style, equals(PaintingStyle.fill));
+    expect(sliderBox, paints..circle(style: PaintingStyle.fill));
+    expect(sliderBox, isNot(paints..circle()..circle()));
     await tester.pumpWidget(buildApp(true));
-    expect(getThumbPaint().style, equals(PaintingStyle.stroke));
+    expect(sliderBox, paints..circle(style: PaintingStyle.stroke));
+    expect(sliderBox, isNot(paints..circle()..circle()));
   });
 
   testWidgets('Slider can tap in vertical scroller',

packages/flutter/test/painting/text_painter_test.dart

@@ -7,8 +7,6 @@ import 'dart:ui' as ui;
 import 'package:flutter/painting.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-import '../rendering/mock_canvas.dart';
-
 void main() {
   test("TextPainter caret test", () {
     TextPainter painter = new TextPainter();
@@ -32,6 +30,6 @@ void main() {
 
   test("TextPainter error test", () {
     TextPainter painter = new TextPainter();
-    expect(() { painter.paint(new MockCanvas(), Offset.zero); }, throwsFlutterError);
+    expect(() { painter.paint(null, Offset.zero); }, throwsFlutterError);
   });
 }