fix #30094, assert should work with a function

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart

Index: pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
index 4a8670bd65178dbd7914982df4ac822520dfaddc..6ae616dab655b3d28fb80e5d1001095487fff81f 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
@@ -461,7 +461,6 @@ registerExtension(jsType, dartExtType) => JS(
     let originalDesc = $getOwnPropertyDescriptor($dartExtType, sigF);
     if (originalDesc === void 0) return;
     let originalSigFn = originalDesc.get;
-    $assert_(originalSigFn);
     $defineMemoizedGetter($jsType, sigF, originalSigFn);
   }
   updateSig($_methodSig);