Revert "[vm] Prevent non-fatal errors from causing timers to be dropped."

tests/isolate/non_fatal_exception_in_timer_callback_test.dart

-// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-import 'dart:async';
-import 'dart:isolate';
-import 'dart:io';
-
-main() async {
-  Isolate.current.setErrorsFatal(false);
-
-  new Timer(const Duration(milliseconds: 10), () {
-    print("Timer 1");
-
-    // This unhandled exception should not prevent the second timer from firing.
-    throw "Oh no!";
-  });
-
-  new Timer.periodic(const Duration(milliseconds: 20), (_) {
-    print("Timer 2");
-    exit(0);
-  });
-
-  sleep(const Duration(milliseconds: 30)); //# sleep: ok
-  // With sleep: both timers are due at the same wakeup event.
-  // Without sleep: the timers get separate wakeup events.
-}