Remove isolate pointer from context objects.

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 41244)
+++ runtime/vm/object.h	(working copy)
@@ -4067,14 +4067,10 @@
  public:
   RawContext* parent() const { return raw_ptr()->parent_; }
   void set_parent(const Context& parent) const {
-    ASSERT(parent.IsNull() || parent.isolate() == Isolate::Current());
     StorePointer(&raw_ptr()->parent_, parent.raw());
   }
   static intptr_t parent_offset() { return OFFSET_OF(RawContext, parent_); }
 
-  Isolate* isolate() const { return raw_ptr()->isolate_; }
-  static intptr_t isolate_offset() { return OFFSET_OF(RawContext, isolate_); }
-
   intptr_t num_variables() const { return raw_ptr()->num_variables_; }
   static intptr_t num_variables_offset() {
     return OFFSET_OF(RawContext, num_variables_);
@@ -4114,10 +4110,6 @@
     return &raw_ptr()->data()[context_index];
   }
 
-  void set_isolate(Isolate* isolate) const {
-    StoreNonPointer(&raw_ptr()->isolate_, isolate);
-  }
-
   void set_num_variables(intptr_t num_variables) const {
     StoreNonPointer(&raw_ptr()->num_variables_, num_variables);
   }