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

runtime/lib/timer_impl.dart

Index: runtime/lib/timer_impl.dart
diff --git a/runtime/lib/timer_impl.dart b/runtime/lib/timer_impl.dart
index 9a2d3ffbc1ac31eb29c984055da603f0cf609805..8c0a670abdfdaedafdb94b18d33dc589c7e4e2fa 100644
--- a/runtime/lib/timer_impl.dart
+++ b/runtime/lib/timer_impl.dart
@@ -348,8 +348,9 @@ class _Timer implements Timer {
     // callbacks will be enqueued now and notified in the next spin at the
     // earliest.
     _handlingCallbacks = true;
+    var i = 0;
     try {
-      for (var i = 0; i < pendingTimers.length; i++) {
+      for (; i < pendingTimers.length; i++) {
         // Next pending timer.
         var timer = pendingTimers[i];
         timer._indexOrNext = null;
@@ -378,6 +379,12 @@ class _Timer implements Timer {
       }
     } finally {
       _handlingCallbacks = false;
+      // Re-queue timers we didn't get to.
+      for (i++; i < pendingTimers.length; i++) {
+        var timer = pendingTimers[i];
+        timer._enqueue();
+      }
+      _notifyEventHandler();

[zra, 2017/08/16 21:46:56]

Would it be okay to only notify the event handler if an event needed to be
requeued, or do you need to do it every time?

[rmacnak, 2017/08/16 21:55:58]

Even if no timers needs to be requeued, there may be an existing timer we need
to set a new wakeup for. Compare the use of _notifyEventHandler() in
_handleMessage, which gets skipped there is an unhandled exception.

     }
   }