[serializer] allow duplicate API external references.

src/snapshot/serializer-common.cc

 // Copyright 2016 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.
 
 #include "src/snapshot/serializer-common.h"
 
 #include "src/external-reference-table.h"
 #include "src/ic/stub-cache.h"
 #include "src/list-inl.h"
 
 namespace v8 {
 namespace internal {
 
 ExternalReferenceEncoder::ExternalReferenceEncoder(Isolate* isolate) {
   map_ = isolate->external_reference_map();
 #ifdef DEBUG
   table_ = ExternalReferenceTable::instance(isolate);
 #endif  // DEBUG
   if (map_ != nullptr) return;
   map_ = new AddressToIndexHashMap();
   ExternalReferenceTable* table = ExternalReferenceTable::instance(isolate);
   for (uint32_t i = 0; i < table->size(); ++i) {
     Address addr = table->address(i);
+    // Ignore duplicate API references.
+    if (table->is_api_reference(i) && !map_->Get(addr).IsNothing()) continue;
     DCHECK(map_->Get(addr).IsNothing());
     map_->Set(addr, i);
     DCHECK(map_->Get(addr).IsJust());
   }
   isolate->set_external_reference_map(map_);
 }
 
 uint32_t ExternalReferenceEncoder::Encode(Address address) const {
   Maybe<uint32_t> maybe_index = map_->Get(address);
   if (maybe_index.IsNothing()) {
@@ skipping 50 matching lines @@
     List<AccessorInfo*>* accessor_infos) {
   // Restore wiped accessor infos.
   for (AccessorInfo* info : *accessor_infos) {
     Foreign::cast(info->js_getter())
         ->set_foreign_address(info->redirected_getter());
   }
 }
 
 }  // namespace internal
 }  // namespace v8