Second version of support for specifying an unhandled exception callback

runtime/tests/vm/dart/isolate_unhandled_exception_uri_helper.dart

+import 'dart:isolate';
+
+// Isolate script that throws an uncaught exception, which is caught by an
+// uncaught exception handler.
+
+void main() {
+  port.receive((message, replyTo) {
+    if (message == 'throw exception') {
+      throw new RuntimeError('ignore this exception');
+    }
+    replyTo.call('hello');
+    port.close();
+  });
+}
+
+bool _unhandledExceptionCallback(IsolateUnhandledException e) {
+  return e.source.message == 'ignore this exception';
+}