Object.oberve: assertions to narrow down flaky crashes with array length mutation.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 6a1542280d020fa59b6abfca431361c5f573b3cf..c0cfebbb138f241e68ac7bdd51385930b5c0d14d 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -4822,6 +4822,10 @@ class Map: public HeapObject {
   inline void set_elements_kind(ElementsKind elements_kind) {
     ASSERT(elements_kind < kElementsKindCount);
     ASSERT(kElementsKindCount <= (1 << kElementsKindBitCount));
+    ASSERT(!is_observed() ||
+           elements_kind == DICTIONARY_ELEMENTS ||
+           elements_kind == NON_STRICT_ARGUMENTS_ELEMENTS ||
+           IsExternalArrayElementsKind(elements_kind));
     set_bit_field2((bit_field2() & ~kElementsKindMask) |
         (elements_kind << kElementsKindShift));
     ASSERT(this->elements_kind() == elements_kind);