Remove an error that is caught at compile time in Dart 2 (#15797)

diff --git a/packages/flutter/test/painting/text_style_test.dart b/packages/flutter/test/painting/text_style_test.dart
index 1e20775..044cf23 100644
--- a/packages/flutter/test/painting/text_style_test.dart
+++ b/packages/flutter/test/painting/text_style_test.dart
@@ -23,7 +23,6 @@
       fontWeight: FontWeight.w800,
       height: 123.0,
     );
-    expect(() { s1.fontFamily = 'test'; }, throwsA(const isInstanceOf<Error>())); // ignore: ASSIGNMENT_TO_FINAL
     expect(s1.fontFamily, isNull);
     expect(s1.fontSize, 10.0);
     expect(s1.fontWeight, FontWeight.w800);