fix #30330, don't ignore cast failures in DDC for language/lib tests

tools/testing/dart/browser_test.dart

Index: tools/testing/dart/browser_test.dart
diff --git a/tools/testing/dart/browser_test.dart b/tools/testing/dart/browser_test.dart
index b1a244badebdc64f2d60ab4b5e37f9ed5e219cb3..2fb935ba4acbeb0aa675934adc905f37410b8a84 100644
--- a/tools/testing/dart/browser_test.dart
+++ b/tools/testing/dart/browser_test.dart
@@ -84,15 +84,11 @@ window.ddcSettings = {
 <script type="text/javascript">
 requirejs(["$testName", "dart_sdk", "async_helper"],
     function($testName, dart_sdk, async_helper) {  
-  function finish() {
-    // dev_compiler's test runner (language_test.js) uses this to notify the
-    // test results, but it isn't needed for test.dart.
-  }
+  dart_sdk.dart.ignoreWhitelistedErrors(false);
   
   // TODO(rnystrom): This uses DDC's forked version of async_helper. Unfork
   // these packages when possible.
-  async_helper.async_helper.asyncTestInitialize(finish);
-  
+  async_helper.async_helper.asyncTestInitialize(function() {});
   dart_sdk._isolate_helper.startRootIsolate(function() {}, []);
   dartMainRunner($testName.$testName.main);
 });