Changed the test a bit to ensure dart:io is not included as dartium cannot import dart:io

tests/vm/dart/isolate_unhandled_exception_uri_helper.dart

Index: tests/vm/dart/isolate_unhandled_exception_uri_helper.dart
===================================================================
--- tests/vm/dart/isolate_unhandled_exception_uri_helper.dart	(revision 16197)
+++ tests/vm/dart/isolate_unhandled_exception_uri_helper.dart	(working copy)
@@ -12,6 +12,7 @@
 void main() {
   port.receive((message, replyTo) {
     if (message == 'throw exception') {
+      replyTo.call('throwing exception');
       throw new RuntimeError('ignore this exception');
     }
     replyTo.call('hello');