Add debug checks to catch crashes with WeakCell::cast().

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index 62dad8df706c2b265eab7e43f24e72c07c22c1c0..b893379ed51924c65dd8fc489acb354343546225 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -2569,6 +2569,7 @@ void Isolate::CheckDetachedContextsAfterGC() {
   int new_length = 0;
   for (int i = 0; i < length; i += 2) {
     int mark_sweeps = Smi::cast(detached_contexts->get(i))->value();
+    DCHECK(detached_contexts->get(i + 1)->IsWeakCell());
     WeakCell* cell = WeakCell::cast(detached_contexts->get(i + 1));
     if (!cell->cleared()) {
       detached_contexts->set(new_length, Smi::FromInt(mark_sweeps + 1));
@@ -2582,6 +2583,7 @@ void Isolate::CheckDetachedContextsAfterGC() {
            length - new_length, length);
     for (int i = 0; i < new_length; i += 2) {
       int mark_sweeps = Smi::cast(detached_contexts->get(i))->value();
+      DCHECK(detached_contexts->get(i + 1)->IsWeakCell());
       WeakCell* cell = WeakCell::cast(detached_contexts->get(i + 1));
       if (mark_sweeps > 3) {
         PrintF("detached context 0x%p\n survived %d GCs (leak?)\n",