- Implement a first cut for const String.env in the VM to allow

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
===================================================================
--- runtime/vm/isolate.h	(revision 28841)
+++ runtime/vm/isolate.h	(working copy)
@@ -174,6 +174,13 @@
     return init_callback_data_;
   }
 
+  Dart_EnvironmentCallback environment_callback() const {
+    return environment_callback_;
+  }
+  void set_environment_callback(Dart_EnvironmentCallback value) {
+    environment_callback_ = value;
+  }
+
   Dart_LibraryTagHandler library_tag_handler() const {
     return library_tag_handler_;
   }
@@ -393,6 +400,7 @@
   RawContext* top_context_;
   uword top_exit_frame_info_;
   void* init_callback_data_;
+  Dart_EnvironmentCallback environment_callback_;
   Dart_LibraryTagHandler library_tag_handler_;
   ApiState* api_state_;
   StubCode* stub_code_;