Don't use async loading in gen_snapshot.

runtime/bin/dartutils.cc

Index: runtime/bin/dartutils.cc
diff --git a/runtime/bin/dartutils.cc b/runtime/bin/dartutils.cc
index 232dcc4510ae893ba7d8c1182b198a01c07d6214..25aa2f2f8eb8e155e0b6cb672f2b7c31ae03b723 100644
--- a/runtime/bin/dartutils.cc
+++ b/runtime/bin/dartutils.cc
@@ -594,14 +594,6 @@ void FUNCTION_NAME(Builtin_DoneLoading)(Dart_NativeArguments args) {
 }
 
 
-Dart_Handle DartUtils::LoadSourceAsync(Dart_Handle library,
-                                       Dart_Handle url,
-                                       Dart_LibraryTag tag,
-                                       Dart_Handle builtin_lib) {
-  return LoadDataAsync_Invoke(Dart_NewInteger(tag), url, library, builtin_lib);
-}
-
-
 Dart_Handle DartUtils::PrepareForScriptLoading(const char* package_root,
                                                Dart_Handle builtin_lib) {
   // First ensure all required libraries are available.