Quick fix for shader warm up (#28951)

Fix issues in https://github.com/flutter/flutter/pull/28687

packages/flutter/lib/src/painting/shader_warm_up.dart

@@ -67,8 +67,6 @@ abstract class ShaderWarmUp {
   /// Trigger draw operations on a given canvas to warm up GPU shader
   /// compilation cache.
   ///
-  /// Parameter [image] is to be used for drawImage related operations.
-  ///
   /// To decide which draw operations to be added to your custom warm up
   /// process, try capture an skp using `flutter screenshot --observatory-
   /// port=<port> --type=skia` and analyze it with https://debugger.skia.org.
@@ -90,7 +88,7 @@ abstract class ShaderWarmUp {
     final ui.Picture picture = recorder.endRecording();
     final TimelineTask shaderWarmUpTask = TimelineTask();
     shaderWarmUpTask.start('Warm-up shader');
-    picture.toImage(size.width.ceil(), size.height.ceil()).then((ui.Image _) {
+    picture.toImage(size.width.ceil(), size.height.ceil()).then((ui.Image image) {
       shaderWarmUpTask.finish();
     });
   }
@@ -184,10 +182,9 @@ class DefaultShaderWarmUp extends ShaderWarmUp {
       ..layout(const ui.ParagraphConstraints(width: 60.0));
     canvas.drawParagraph(paragraph, const ui.Offset(20.0, 20.0));
 
-
     // Construct an image for drawImage related operations
-    const int imageWidth = 40;
-    const int imageHeight = 40;
+    const int imageWidth = 10;
+    const int imageHeight = 10;
     final Uint8List pixels = Uint8List.fromList(List<int>.generate(
       imageWidth * imageHeight * 4,
           (int i) => i % 4 < 2 ? 0x00 : 0xFF,  // opaque blue
@@ -201,9 +198,9 @@ class DefaultShaderWarmUp extends ShaderWarmUp {
       final ui.Rect srcRect = ui.Rect.fromLTWH(0.0, 0.0, image.width.toDouble(), image.height.toDouble());
       canvas.drawImage(image, const ui.Offset(20.0, 20.0), ui.Paint());
       canvas.translate(80.0, 0.0);
-      canvas.drawImageRect(image, srcRect, ui.Rect.fromLTWH(20.0, 20.0, 20.0, 20.0), paints[0]);
+      canvas.drawImageRect(image, srcRect, ui.Rect.fromLTWH(20.0, 20.0, imageWidth * 0.6, imageWidth * 0.6), paints[0]);
       canvas.translate(80.0, 0.0);
-      canvas.drawImageRect(image, srcRect, ui.Rect.fromLTWH(10.0, 10.0, 60.0, 60.0), paints[0]);
+      canvas.drawImageRect(image, srcRect, ui.Rect.fromLTWH(10.0, 10.0, imageWidth * 1.5, imageWidth * 1.5), paints[0]);
       canvas.restore();
       completer.complete();
     });