Fix function subtyping crash

tool/input_sdk/private/ddc_runtime/types.dart

Index: tool/input_sdk/private/ddc_runtime/types.dart
diff --git a/tool/input_sdk/private/ddc_runtime/types.dart b/tool/input_sdk/private/ddc_runtime/types.dart
index e4ad0353d542a21eb67cac099129bff82a8bde5f..3ed88b844560dfadb29f8175ab80618762d39c1a 100644
--- a/tool/input_sdk/private/ddc_runtime/types.dart
+++ b/tool/input_sdk/private/ddc_runtime/types.dart
@@ -496,6 +496,10 @@ isFunctionSubtype(ft1, ft2, covariant) => JS('', '''(() => {
     return true;
   }
 
+  if ($ft1 === $Function) {
+    return false;
+  }
+
   let ret1 = $ft1.returnType;
   let ret2 = $ft2.returnType;