[web] Fix transform not invalidating path bounds causing debugValidate failure

lib/web_ui/lib/src/engine/html/path/path_ref.dart

@@ -42,7 +42,9 @@ class PathRef {
   static const int kInitialPointsCapacity = 8;
   static const int kInitialVerbsCapacity = 8;
 
+  /// Bounds of points that define path.
   ui.Rect? fBounds;
+  /// Computed tight bounds of path (may exclude curve control points).
   ui.Rect? cachedBounds;
   int _fPointsCapacity = 0;
   int _fPointsLength = 0;
@@ -730,6 +732,7 @@ class PathRef {
     fIsRRect = false;
     fIsRect = false;
     cachedBounds = null;
+    fBoundsIsDirty = true;
   }
 
   void setIsOval(bool isOval, bool isCCW, int start) {

lib/web_ui/test/path_test.dart

@@ -536,5 +536,19 @@ void testMain() {
       end = iter.skipToNextContour();
       expect(start, end);
     });
+
+    /// Regression test for https://github.com/flutter/flutter/issues/68702.
+    test('Path should return correct bounds after transform', () {
+      final Path path1 = Path()
+        ..moveTo(100, 100)
+        ..lineTo(200, 100)
+        ..lineTo(150, 200)
+        ..close();
+      final SurfacePath path2 = Path.from(path1) as SurfacePath;
+      Rect bounds = path2.pathRef.getBounds();
+      SurfacePath transformedPath = path2.transform(
+          Matrix4.identity().scaled(0.5, 0.5).toFloat64());
+      expect(transformedPath.pathRef.getBounds(), isNot(bounds));
+    });
   });
 }