Replace linked list of loaded libraries

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 7922)
+++ runtime/vm/object.h	(working copy)
@@ -1770,11 +1770,10 @@
 
   void Register() const;
 
-  RawLibrary* next_registered() const { return raw_ptr()->next_registered_; }
-
   RawString* DuplicateDefineErrorString(const String& entry_name,
                                         const Library& conflicting_lib) const;
   static RawLibrary* LookupLibrary(const String& url);
+  static RawLibrary* GetLibrary(intptr_t index);
   static RawString* CheckForDuplicateDefinition();
   static bool IsKeyUsed(intptr_t key);