Fix web_ui warnings flagged by the next Dart analyzer update (flutter#20524)

Author: jason-simmons

lib/web_ui/lib/src/engine/compositor/shader.dart

@@ -173,7 +173,7 @@ class CkImageShader extends CkShader implements ui.ImageShader {
   SkShader createDefault() => _skImage.skImage.makeShader(
     toSkTileMode(tileModeX),
     toSkTileMode(tileModeY),
-    matrix4 != null ? toSkMatrixFromFloat64(matrix4) : null,
+    toSkMatrixFromFloat64(matrix4),
   );
 
   @override

lib/web_ui/lib/src/engine/surface/scene_builder.dart

@@ -78,9 +78,6 @@ class SurfaceSceneBuilder implements ui.SceneBuilder {
     Float64List matrix4, {
     ui.TransformEngineLayer? oldLayer,
   }) {
-    if (matrix4 == null) { // ignore: unnecessary_null_comparison
-      throw ArgumentError('"matrix4" argument cannot be null');
-    }
     if (matrix4.length != 16) {
       throw ArgumentError('"matrix4" must have 16 entries.');
     }

lib/web_ui/test/engine/surface/scene_builder_test.dart

@@ -56,7 +56,8 @@ void main() {
       testLayerLifeCycle((SceneBuilder sceneBuilder, EngineLayer oldLayer) {
         return sceneBuilder.pushClipRRect(
             RRect.fromLTRBR(10, 20, 30, 40, const Radius.circular(3)),
-            oldLayer: oldLayer);
+            oldLayer: oldLayer,
+            clipBehavior: Clip.none);
       }, () {
         return '''
 <s>

lib/web_ui/test/text/measurement_test.dart

@@ -1157,5 +1157,6 @@ EngineLineMetrics line(
     lineNumber: lineNumber,
     left: left,
     endIndexWithoutNewlines: -1,
+    widthWithTrailingSpaces: width,
   );
 }