[ddc] Add temporary fix for typeAcceptsNull()

The CFE is evaluating the constant as if strong mode is always
enabled.

This is a temporary work around and we should be able to access
the mode flag here in a future change.

Change-Id: I28842a692183c8da7aa42086a034f5388af83af5
Issue: dart-lang#41657
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144815
Reviewed-by: Sigmund Cherem <[email protected]>
Commit-Queue: Nicholas Shahan <[email protected]>

Author: nshahan

sdk_nnbd/lib/_internal/js_dev_runtime/patch/internal_patch.dart

@@ -10,9 +10,11 @@ import 'dart:_interceptors' show JSArray;
 import 'dart:_foreign_helper' show JS;
 import 'dart:_runtime' as dart;
 
+// TODO(41657) Make a constant when CFE evaluates in the null safety mode.
+var isLegacySubtyping = const <Null>[] is List<int>;
+
 @patch
 bool typeAcceptsNull<T>() {
-  const isLegacySubtyping = <Null>[] is List<int>;
   return isLegacySubtyping || null is T;
 }