fixes

Author: goderbauer

lib/web_ui/dev/test_platform.dart

@@ -1004,7 +1004,7 @@ class BrowserManager {
           final String pathToTest = p.dirname(path);
 
           final String mapPath = p.join(
-            _sourceMapDirectory!.path,
+            _sourceMapDirectory.path,
             pathToTest,
             sourceMapFileName
           );

lib/web_ui/lib/src/engine/canvaskit/layer.dart

@@ -406,8 +406,7 @@ class ImageFilterEngineLayer extends ContainerLayer
     prerollContext.mutatorsStack.pushTransform(transform);
     final ui.Rect childPaintBounds =
         prerollChildren(prerollContext, childMatrix);
-    (_filter as CkManagedSkImageFilterConvertible)
-        .imageFilter((SkImageFilter filter) {
+    _filter.imageFilter((SkImageFilter filter) {
       paintBounds =
           rectFromSkIRect(filter.getOutputBounds(toSkRect(childPaintBounds)));
     });

lib/web_ui/test/common/fake_asset_manager.dart

@@ -81,7 +81,7 @@ class FakeAssetScope {
       return fetcher();
     }
     if (_parent != null) {
-      return _parent!.getAssetData(assetKey);
+      return _parent.getAssetData(assetKey);
     }
     return null;
   }