Second step towards loading the core library scripts directly from the sources

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
===================================================================
--- runtime/vm/isolate.h	(revision 22375)
+++ runtime/vm/isolate.h	(working copy)
@@ -381,9 +381,11 @@
   }
 
   static void SetFileCallbacks(Dart_FileOpenCallback file_open,
+                               Dart_FileReadCallback file_read,
                                Dart_FileWriteCallback file_write,
                                Dart_FileCloseCallback file_close) {
     file_open_callback_ = file_open;
+    file_read_callback_ = file_read;
     file_write_callback_ = file_write;
     file_close_callback_ = file_close;
   }
@@ -391,6 +393,9 @@
   static Dart_FileOpenCallback file_open_callback() {
     return file_open_callback_;
   }
+  static Dart_FileReadCallback file_read_callback() {
+    return file_read_callback_;
+  }
   static Dart_FileWriteCallback file_write_callback() {
     return file_write_callback_;
   }
@@ -520,6 +525,7 @@
   static Dart_IsolateUnhandledExceptionCallback unhandled_exception_callback_;
   static Dart_IsolateShutdownCallback shutdown_callback_;
   static Dart_FileOpenCallback file_open_callback_;
+  static Dart_FileReadCallback file_read_callback_;
   static Dart_FileWriteCallback file_write_callback_;
   static Dart_FileCloseCallback file_close_callback_;
   static Dart_IsolateInterruptCallback vmstats_callback_;