Clean up the test srapper code so that it does not use reportTestError, and made that private.

lib/unittest/html_config.dart

Index: lib/unittest/html_config.dart
===================================================================
--- lib/unittest/html_config.dart	(revision 10299)
+++ lib/unittest/html_config.dart	(working copy)
@@ -21,11 +21,8 @@
   EventListener _onErrorClosure;
 
   void onInit() {
-    _onErrorClosure = (e) {
-      // TODO(vsm): figure out how to expose the stack trace here
-      // Currently e.message works in dartium, but not in dartc.
-      reportTestError('(DOM callback has errors) Caught ${e}', '');
-    };
+    _onErrorClosure =
+        (e) => handleExternalError(e, '(DOM callback has errors)');
   }
 
   void onStart() {