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

runtime/bin/builtin_gen_snapshot.cc

Index: runtime/bin/builtin_gen_snapshot.cc
diff --git a/runtime/bin/builtin.cc b/runtime/bin/builtin_gen_snapshot.cc
similarity index 83%
copy from runtime/bin/builtin.cc
copy to runtime/bin/builtin_gen_snapshot.cc
index f6d297e9c8435400cc0f83f30b2803f9a150fdfd..fd64f46bd9cfef863c6eda7adeba2be7ecad3fa4 100644
--- a/runtime/bin/builtin.cc
+++ b/runtime/bin/builtin_gen_snapshot.cc
@@ -30,6 +30,13 @@ Dart_Handle Builtin::Source(BuiltinLibraryId id) {
 }
 
 
+Dart_NativeFunction Builtin::NativeLookup(Dart_Handle name,
+                                          int argument_count) {
+  UNREACHABLE();
+  return NULL;
+}
+
+
 void Builtin::SetNativeResolver(BuiltinLibraryId id) {
   UNREACHABLE();
 }
@@ -45,7 +52,10 @@ Dart_Handle Builtin::LoadAndCheckLibrary(BuiltinLibraryId id) {
     library = Dart_LoadLibrary(url, Source(id));
     if (!Dart_IsError(library) && (builtin_libraries_[id].has_natives_)) {
       // Setup the native resolver for built in library functions.
-      DART_CHECK_VALID(Dart_SetNativeResolver(library, NativeLookup));
+      // Looks up native functions only in libdart_builtin, not libdart_io.
+      // This is for use in the snapshot generator, which should be
+      // independent of most of the dart:io C++ code.
+      DART_CHECK_VALID(Dart_SetNativeResolver(library, BuiltinNativeLookup));
     }
     if (builtin_libraries_[id].patch_url_ != NULL) {
       ASSERT(builtin_libraries_[id].patch_source_ != NULL);