Fix context unchaining (issue 5991015).

runtime/vm/scopes.h

 // 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.
 
 #ifndef VM_SCOPES_H_
 #define VM_SCOPES_H_
 
 #include "vm/allocation.h"
 #include "vm/assembler.h"
 #include "vm/growable_array.h"
@@ skipping 148 matching lines @@
 
   LocalScope* parent() const { return parent_; }
   LocalScope* child() const { return child_; }
   LocalScope* sibling() const { return sibling_; }
   int function_level() const { return function_level_; }
   int loop_level() const { return loop_level_; }
 
   // Check if this scope is nested within the passed in scope.
   bool IsNestedWithin(LocalScope* scope) const;
 
+  // The context level is only set in a scope that is either the owner scope of
+  // a captured variable or that is the owner scope of a context.
   bool HasContextLevel() const {
     return context_level_ != kUnitializedContextLevel_;
   }
   int context_level() const {
     ASSERT(HasContextLevel());
     return context_level_;
   }
   void set_context_level(int context_level) {
     ASSERT(!HasContextLevel());
     ASSERT(context_level != kUnitializedContextLevel_);
@@ skipping 113 matching lines @@
   intptr_t end_token_index_;  // Token index of end of scope.
   GrowableArray<LocalVariable*> variables_;
   GrowableArray<SourceLabel*> labels_;
 
   DISALLOW_COPY_AND_ASSIGN(LocalScope);
 };
 
 }  // namespace dart
 
 #endif  // VM_SCOPES_H_