Reapply "[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.
+}