Make CTX allocatable by the register allocator.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
===================================================================
--- runtime/vm/isolate.cc	(revision 41393)
+++ runtime/vm/isolate.cc	(working copy)
@@ -401,7 +401,6 @@
       terminate_capability_(0),
       heap_(NULL),
       object_store_(NULL),
-      top_context_(Context::null()),
       top_exit_frame_info_(0),
       init_callback_data_(NULL),
       environment_callback_(NULL),
@@ -461,7 +460,6 @@
       terminate_capability_(0),
       heap_(NULL),
       object_store_(NULL),
-      top_context_(Context::null()),
       top_exit_frame_info_(0),
       init_callback_data_(NULL),
       environment_callback_(NULL),
@@ -1130,9 +1128,6 @@
     api_state()->VisitObjectPointers(visitor, visit_prologue_weak_handles);
   }
 
-  // Visit the top context which is stored in the isolate.
-  visitor->VisitPointer(reinterpret_cast<RawObject**>(&top_context_));
-
   // Visit the current tag which is stored in the isolate.
   visitor->VisitPointer(reinterpret_cast<RawObject**>(&current_tag_));