SnapshotCreator: start from existing snapshot if we have one

src/snapshot/deserializer.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/deserializer.h"
 
 #include "src/api.h"
 #include "src/assembler-inl.h"
 #include "src/bootstrapper.h"
 #include "src/external-reference-table.h"
@@ skipping 58 matching lines @@
 }
 
 void Deserializer::Initialize(Isolate* isolate) {
   DCHECK_NULL(isolate_);
   DCHECK_NOT_NULL(isolate);
   isolate_ = isolate;
   DCHECK_NULL(external_reference_table_);
   external_reference_table_ = ExternalReferenceTable::instance(isolate);
   CHECK_EQ(magic_number_,
            SerializedData::ComputeMagicNumber(external_reference_table_));
+  // The current isolate must have at least as many API-provided external
+  // references as the to-be-deserialized snapshot expects and refers to.
+  CHECK_LE(num_extra_references_,
+           SerializedData::GetExtraReferences(external_reference_table_));
 }
 
 void Deserializer::Deserialize(Isolate* isolate) {
   Initialize(isolate);
   if (!ReserveSpace()) V8::FatalProcessOutOfMemory("deserializing context");
   // No active threads.
   DCHECK_NULL(isolate_->thread_manager()->FirstThreadStateInUse());
   // No active handles.
   DCHECK(isolate_->handle_scope_implementer()->blocks()->is_empty());
   // Partial snapshot cache is not yet populated.
@@ skipping 772 matching lines @@
 
       default:
         CHECK(false);
     }
   }
   CHECK_EQ(limit, current);
   return true;
 }
 }  // namespace internal
 }  // namespace v8