v8: Prepare for swallowed rejection hook.

This is done in preparation for landing

  https://chromium-review.googlesource.com/c/v8/v8/+/1126099

on the V8 side, which extends the existing PromiseRejectEvent mechanism
with new hooks for reject/resolve after a Promise was previously
resolved already.

Refs: nodejs/promises-debugging#8
Design: https://goo.gl/2stLUY

src/node.cc

@@ -872,7 +872,9 @@ void PromiseRejectCallback(PromiseRejectMessage message) {
     callback = env->promise_reject_handled_function();
     value = Undefined(isolate);
   } else {
-    UNREACHABLE();
+    // Ignore reject/resolve after resolved events for now.
+    // TODO(benjamingr) warn about misuse of Promise constructor.
+    return;
   }
 
   Local<Value> args[] = { promise, value };