[cupertino_ui] Work around dart2wasm optional parameter inference bug in checkbox_test.dart (#12956)

Adds a workaround in packages/cupertino_ui/test/checkbox_test.dart for a dart2wasm regression introduced in https://github.com/dart-lang/sdk/commit/6eb8d5c99d649b27ad338c123909dda9cc082cec ([dart2wasm] Use inferred type also for optional parameters) to unblock the Flutter-to-packages roller (https://github.com/flutter/packages/pull/12904).

### Background
When running the full cupertino_ui test suite on WebAssembly, other tests in the bundle keep _RoundedSuperellipseToCircleBorder.copyWith and _RoundedRectangleToCircleBorder.copyWith alive in the OutlinedBorder.copyWith dispatch table selector. Because all call sites targeting those two implementations pass non-null doubles for their optional circularity and eccentricity parameters, Type Flow Analysis (TFA) infers non-nullable double (unboxed f64) for those parameter slots in the shared selector signature (which is also used by RoundedRectangleBorder.copyWith). When _CheckboxPainter._drawBox calls shape.copyWith(side: side) on RoundedRectangleBorder, dart2wasm pushes the selector's default value (NullConstant) into the unboxed f64 parameter slot, causing ConstantInstantiator to emit an unconditional WebAssembly unreachable instruction and hanging the test suite.

### Fix
Calling copyWith() without arguments on lerped OutlinedBorder instances in checkbox_test.dart forces TFA to flow Null into the circularity and eccentricity parameter slots of the OutlinedBorder.copyWith selector. This causes dart2wasm to keep those parameters boxed and nullable instead of emitting unreachable in _CheckboxPainter._drawBox, while keeping all checkbox tests enabled.

Workaround for https://github.com/flutter/flutter/issues/193021
Workaround for https://github.com/dart-lang/sdk/issues/64326

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packages/cupertino_ui/test/checkbox_test.dart

diff --git a/packages/cupertino_ui/test/checkbox_test.dart b/packages/cupertino_ui/test/checkbox_test.dart
index baf115b..55afffb 100644
--- a/packages/cupertino_ui/test/checkbox_test.dart
+++ b/packages/cupertino_ui/test/checkbox_test.dart
@@ -16,6 +16,18 @@
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
+  // Workaround for https://github.com/dart-lang/sdk/issues/64326:
+  // Ensure TFA infers nullable `circularity` and `eccentricity` parameters on
+  // `_ShapeToCircleBorder.copyWith` implementations in the `OutlinedBorder.copyWith`
+  // dispatch selector so dart2wasm does not miscompile `shape.copyWith(side: side)`
+  // in `_CheckboxPainter._drawBox` into `unreachable`.
+  // TODO(Piinks): Restore original test when https://github.com/dart-lang/sdk/issues/64326 is resolved.
+  (ShapeBorder.lerp(const RoundedRectangleBorder(), const CircleBorder(), 0.5)! as OutlinedBorder)
+      .copyWith();
+  (ShapeBorder.lerp(const RoundedSuperellipseBorder(), const CircleBorder(), 0.5)!
+          as OutlinedBorder)
+      .copyWith();
+
   setUp(() {
     debugResetSemanticsIdCounter();
   });