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

runtime/bin/builtin.cc

Index: runtime/bin/builtin.cc
diff --git a/runtime/bin/builtin.cc b/runtime/bin/builtin.cc
index f6d297e9c8435400cc0f83f30b2803f9a150fdfd..f9218dfa06975c0fb829a44ad5c4379cab4f96b9 100644
--- a/runtime/bin/builtin.cc
+++ b/runtime/bin/builtin.cc
@@ -8,7 +8,7 @@
 
 #include "bin/builtin.h"
 #include "bin/dartutils.h"
-
+#include "bin/io_natives.h"
 
 Builtin::builtin_lib_props Builtin::builtin_libraries_[] = {
   /* { url_, source_, patch_url_, patch_source_, has_natives_ } */
@@ -29,6 +29,15 @@ Dart_Handle Builtin::Source(BuiltinLibraryId id) {
   return DartUtils::NewString(builtin_libraries_[id].source_);
 }
 
+/**
+ * Looks up native functions in both libdart_builtin and libdart_io.
+ */
+Dart_NativeFunction Builtin::NativeLookup(Dart_Handle name,
+                                          int argument_count) {
+  Dart_NativeFunction result = BuiltinNativeLookup(name, argument_count);
+  if (result != NULL) return result;
+  return IONativeLookup(name, argument_count);
+}
 
 void Builtin::SetNativeResolver(BuiltinLibraryId id) {
   UNREACHABLE();