Tree-shaker: drop imports/exports in a way that doesn't break the vm-service.

runtime/vm/raw_object_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/native_entry.h"
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/snapshot.h"
 #include "vm/stub_code.h"
 #include "vm/symbols.h"
@@ skipping 1282 matching lines @@
                          reader->Read<bool>());
   prefix.StoreNonPointer(&prefix.raw_ptr()->is_loaded_, reader->Read<bool>());
 
   // Set all the object fields.
   READ_OBJECT_FIELDS(prefix,
                      prefix.raw()->from(),
                      prefix.raw()->to_snapshot(kind),
                      kAsReference);
   if (kind == Snapshot::kAppNoJIT) {
     prefix.StorePointer(&prefix.raw_ptr()->imports_,
-                        Array::null());
+                        Object::empty_array().raw());
   }
   prefix.StorePointer(&prefix.raw_ptr()->dependent_code_,
                       Array::null());
 
   return prefix.raw();
 }
 
 
 void RawLibraryPrefix::WriteTo(SnapshotWriter* writer,
                                intptr_t object_id,
@@ skipping 2466 matching lines @@
     // We do not allow objects with native fields in an isolate message.
     writer->SetWriteException(Exceptions::kArgument,
                               "Illegal argument in isolate message"
                               " : (object is a UserTag)");
   } else {
     UNREACHABLE();
   }
 }
 
 }  // namespace dart