Consider Contexts to be user objects for purposes of Obervatory's dominator analysis.

runtime/vm/object_graph.cc

 // Copyright (c) 2014, 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/object_graph.h"
 
 #include "vm/dart.h"
 #include "vm/growable_array.h"
 #include "vm/isolate.h"
 #include "vm/object.h"
@@ skipping 532 matching lines @@
         ptr_writer_(isolate, stream, roots == ObjectGraph::kUser),
         roots_(roots),
         count_(0) {}
 
   virtual Direction VisitObject(ObjectGraph::StackIterator* it) {
     RawObject* raw_obj = it->Get();
     Thread* thread = Thread::Current();
     REUSABLE_OBJECT_HANDLESCOPE(thread);
     Object& obj = thread->ObjectHandle();
     obj = raw_obj;
-    if ((roots_ == ObjectGraph::kVM) || obj.IsField() || obj.IsInstance()) {
+    if ((roots_ == ObjectGraph::kVM) || obj.IsField() || obj.IsInstance() ||
+        obj.IsContext()) {
       // Each object is a header + a zero-terminated list of its neighbors.
       WriteHeader(raw_obj, raw_obj->Size(), obj.GetClassId(), stream_);
       raw_obj->VisitPointers(&ptr_writer_);
       stream_->WriteUnsigned(0);
       ++count_;
     }
     return kProceed;
   }
 
   intptr_t count() const { return count_; }
@@ skipping 90 matching lines @@
   intptr_t object_count = visitor.count();
   if (roots == kVM) {
     object_count += 1;  // root
   } else {
     object_count += 2;  // root and stack
   }
   return object_count;
 }
 
 }  // namespace dart