Ensure deoptimizing frames have a valid pc marker before deferred materialization.

runtime/vm/deopt_instructions.cc

 // Copyright (c) 2013, 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/deopt_instructions.h"
 
 #include "vm/assembler.h"
 #include "vm/code_patcher.h"
 #include "vm/compiler.h"
 #include "vm/intermediate_language.h"
@@ skipping 648 matching lines @@
   void Execute(DeoptContext* deopt_context, intptr_t* dest_addr) {
     Function& function = Function::Handle(deopt_context->zone());
     function ^= deopt_context->ObjectAt(object_table_index_);
     if (function.IsNull()) {
       *reinterpret_cast<RawObject**>(dest_addr) = deopt_context->is_lazy_deopt()
           ? StubCode::DeoptimizeLazy_entry()->code()
           : StubCode::Deoptimize_entry()->code();
       return;
     }
 
-    *dest_addr = reinterpret_cast<intptr_t>(Object::null());
-    deopt_context->DeferPcMarkerMaterialization(
-        object_table_index_, dest_addr);
+    // We don't always have the Code object for the frame's corresponding
+    // unoptimized code as it may have been collected. Use a stub as the pc
+    // marker until we can recreate that Code object during deferred
+    // materialization to maintain the invariant that Dart frames always have
+    // a pc marker.
+    *reinterpret_cast<RawObject**>(dest_addr) =
+        StubCode::FrameAwaitingMaterialization_entry()->code();
+    deopt_context->DeferPcMarkerMaterialization(object_table_index_, dest_addr);
   }
 
  private:
   intptr_t object_table_index_;
 
   DISALLOW_COPY_AND_ASSIGN(DeoptPcMarkerInstr);
 };
 
 
 // Deoptimization instruction creating a pool pointer for the code of
@@ skipping 627 matching lines @@
                           Smi* offset,
                           TypedData* info,
                           Smi* reason) {
   intptr_t i = index * kEntrySize;
   *offset ^= table.At(i);
   *info ^= table.At(i + 1);
   *reason ^= table.At(i + 2);
 }
 
 }  // namespace dart