Fix background compilation issues when creating snapshots.

runtime/vm/snapshot.cc

Index: runtime/vm/snapshot.cc
diff --git a/runtime/vm/snapshot.cc b/runtime/vm/snapshot.cc
index 8d1fdcf2b5efd3082c30e3287b21dbd0aacaffbd..f7a880d59b5aa3a1ae213ea4ea01f4c7edc4f7d0 100644
--- a/runtime/vm/snapshot.cc
+++ b/runtime/vm/snapshot.cc
@@ -1936,6 +1936,8 @@ FullSnapshotWriter::FullSnapshotWriter(uint8_t** vm_isolate_snapshot_buffer,
 
 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();
   scripts_ = Array::null();
 }