Fix context-allocated variables in stack traces

runtime/vm/debugger.cc

Index: runtime/vm/debugger.cc
===================================================================
--- runtime/vm/debugger.cc	(revision 22061)
+++ runtime/vm/debugger.cc	(working copy)
@@ -339,7 +339,7 @@
 }
 
 
-RawContext* ActivationFrame::GetSavedContext() {
+RawContext* ActivationFrame::GetSavedContext(const Context& ctx) {
   GetVarDescriptors();
   intptr_t var_desc_len = var_descriptors_.Length();
   for (int i = 0; i < var_desc_len; i++) {
@@ -349,8 +349,7 @@
       return reinterpret_cast<RawContext*>(GetLocalVarValue(var_info.index));
     }
   }
-  UNREACHABLE();
-  return Context::null();
+  return ctx.raw();
 }
 
 
@@ -898,10 +897,8 @@
         activation->SetContext(Context::Handle());
         optimized_frame_found = true;
       } else {
-        if (get_saved_context) {
-          ctx = activation->GetSavedContext();
-        }
         activation->SetContext(ctx);
+        ctx = activation->GetSavedContext(ctx);
       }
       stack_trace->AddActivation(activation);
       get_saved_context = activation->function().IsClosureFunction();