Allow IdentityMap to store Smi-0.

src/heap/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 #include <map>
 
@@ skipping 318 matching lines @@
   V(frozen_symbol)                          \
   V(hash_code_symbol)                       \
   V(home_object_symbol)                     \
   V(internal_error_symbol)                  \
   V(intl_impl_object_symbol)                \
   V(intl_initialized_marker_symbol)         \
   V(megamorphic_symbol)                     \
   V(nonexistent_symbol)                     \
   V(nonextensible_symbol)                   \
   V(normal_ic_symbol)                       \
+  V(not_mapped_symbol)                      \
   V(observed_symbol)                        \
   V(premonomorphic_symbol)                  \
   V(promise_combined_deferred_symbol)       \
   V(promise_debug_marker_symbol)            \
   V(promise_has_handler_symbol)             \
   V(promise_on_resolve_symbol)              \
   V(promise_on_reject_symbol)               \
   V(promise_raw_symbol)                     \
   V(promise_status_symbol)                  \
   V(promise_value_symbol)                   \
@@ skipping 2383 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_HEAP_H_