VM: Use current isolate as immediate in per-isolate stubs.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
===================================================================
--- runtime/vm/isolate.h	(revision 40376)
+++ runtime/vm/isolate.h	(working copy)
@@ -114,6 +114,10 @@
     return reinterpret_cast<Isolate*>(Thread::GetThreadLocal(isolate_key));
   }
 
+  static inline uword CurrentAddress() {

[Ivan Posva, 2014/09/18 17:31:12]

This is a rather unfortunate name as the function does not return the address of
Current (see Scavenger::TopAddress for an example).

I will rename this to CurrentAsWord() if we need it.

+    return reinterpret_cast<uword>(Current());
+  }
+
   static void SetCurrent(Isolate* isolate);
 
   static void InitOnce();