Fix context unchaining (issue 5991015).

runtime/vm/scopes.cc

 // Copyright (c) 2011, 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/scopes.h"
 
 #include "vm/ast.h"
 #include "vm/object.h"
 
 namespace dart {
@@ skipping 73 matching lines @@
   ASSERT(variable->is_captured());
   ASSERT(variable->owner()->loop_level() == loop_level());
   if (num_context_variables_ == 0) {
     // This scope will allocate and chain a new context.
     int new_context_level = ((*context_owner) == NULL) ?
         1 : (*context_owner)->context_level() + 1;
     // This scope becomes the current context owner.
     *context_owner = this;
     set_context_level(new_context_level);
   }
+  // The context level in the owner scope of a captured variable indicates at
+  // code generation time how far to walk up the context chain in order to
+  // access the variable from the current context level.
   if (!variable->owner()->HasContextLevel()) {
     ASSERT(variable->owner() != this);
     variable->owner()->set_context_level(context_level());
+  } else {
+    ASSERT(variable->owner()->context_level() == context_level());
   }
   variable->set_index(num_context_variables_++);
 }
 
 
 int LocalScope::AllocateVariables(int first_parameter_index,
                                   int num_parameters,
                                   int first_frame_index,
                                   LocalScope* loop_owner,
                                   LocalScope** context_owner) {
@@ skipping 51 matching lines @@
                                                      frame_index,
                                                      loop_owner,
                                                      &child_context_owner);
     if (child_frame_index < min_frame_index) {
       min_frame_index = child_frame_index;
     }
     // A context allocated at a deeper loop level than the current loop level is
     // not shared between children.
     if ((child_context_owner != *context_owner) &&
         (child_context_owner->loop_level() <= loop_owner->loop_level())) {
-      *context_owner = child_context_owner;
+      *context_owner = child_context_owner;  // Share context between siblings.
     }
     child = child->sibling();
   }
   return min_frame_index;
 }
 
 
 RawLocalVarDescriptors* LocalScope::GetVarDescriptors() {
   GrowableArray<LocalVariable*> vars(8);
   // Variables of each scope are guaranteed to be consecutive elements
@@ skipping 284 matching lines @@
       }
       if (owner()->context_level() == other.owner()->context_level()) {
         return true;
       }
     }
   }
   return false;
 }
 
 }  // namespace dart