Fix full snapshot writer code

runtime/vm/snapshot.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/snapshot.h"
 
 #include "platform/assert.h"
 #include "vm/bootstrap.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart.h"
@@ skipping 1899 matching lines @@
   // into an array so that we can write it out as part of the VM isolate
   // snapshot. We first count the number of script objects, allocate an array
   // and then fill it up with the script objects.
   ScriptVisitor scripts_counter(thread());
   heap()->IterateOldObjects(&scripts_counter);
   intptr_t count = scripts_counter.count();
   scripts_ = Array::New(count, Heap::kOld);
   ScriptVisitor script_visitor(thread(), &scripts_);
   heap()->IterateOldObjects(&script_visitor);
 
-  // Stash the symbol table away for writing and reading into the vm isolate,
-  // and reset the symbol table for the regular isolate so that we do not
-  // write these symbols into the snapshot of a regular dart isolate.
-  symbol_table_ = object_store->symbol_table();
-  Symbols::SetupSymbolTable(isolate());
+  if (vm_isolate_snapshot_buffer != NULL) {
+    // Stash the symbol table away for writing and reading into the vm isolate,
+    // and reset the symbol table for the regular isolate so that we do not
+    // write these symbols into the snapshot of a regular dart isolate.
+    symbol_table_ = object_store->symbol_table();
+    Symbols::SetupSymbolTable(isolate());
+  }
 
   forward_list_ = new ForwardList(thread(), SnapshotWriter::FirstObjectId());
   ASSERT(forward_list_ != NULL);
 
   if (instructions_snapshot_buffer != NULL) {
     instructions_writer_ = new InstructionsWriter(instructions_snapshot_buffer,
                                                   alloc,
                                                   kInitialSize);
   }
 }
 
 
 FullSnapshotWriter::~FullSnapshotWriter() {
   delete forward_list_;
-  // We may run Dart code afterwards, restore the symbol table.
-  isolate()->object_store()->set_symbol_table(symbol_table_);
-  symbol_table_ = Array::null();
+  // We may run Dart code afterwards, restore the symbol table if needed.
+  if (!symbol_table_.IsNull()) {
+    isolate()->object_store()->set_symbol_table(symbol_table_);
+    symbol_table_ = Array::null();
+  }
   scripts_ = Array::null();
 }
 
 
 void FullSnapshotWriter::WriteVmIsolateSnapshot() {
   ASSERT(vm_isolate_snapshot_buffer_ != NULL);
   SnapshotWriter writer(Snapshot::kFull,
                         thread(),
                         vm_isolate_snapshot_buffer_,
                         alloc_,
@@ skipping 710 matching lines @@
   if (setjmp(*jump.Set()) == 0) {
     NoSafepointScope no_safepoint;
     WriteObject(obj.raw());
   } else {
     ThrowException(exception_type(), exception_msg());
   }
 }
 
 
 }  // namespace dart