When building standalone Dart executable, put implementation of dart:io natives in a separate libra…

runtime/bin/builtin.h

Index: runtime/bin/builtin.h
diff --git a/runtime/bin/builtin.h b/runtime/bin/builtin.h
index a46fbdba78e0e4ca08bb51bfc59b6a22f05bea19..b21c9af475ff8127bd60e3e417d5f8f082745f88 100644
--- a/runtime/bin/builtin.h
+++ b/runtime/bin/builtin.h
@@ -42,6 +42,8 @@ class Builtin {
  private:
   static Dart_NativeFunction NativeLookup(Dart_Handle name,
                                           int argument_count);
+  static Dart_NativeFunction BuiltinNativeLookup(Dart_Handle name,
+                                                 int argument_count);
 
   static const char builtin_source_[];
   static const char crypto_source_[];