Don't use async loading in gen_snapshot.

runtime/bin/gen_snapshot.cc

Index: runtime/bin/gen_snapshot.cc
diff --git a/runtime/bin/gen_snapshot.cc b/runtime/bin/gen_snapshot.cc
index 5dd431bb4d4ac530c9c496ebb52ea5cc3cf371f1..ff2d3b877dea73a4c35fa688fa2f6faa5127a185 100644
--- a/runtime/bin/gen_snapshot.cc
+++ b/runtime/bin/gen_snapshot.cc
@@ -351,17 +351,17 @@ static Dart_Handle CreateSnapshotLibraryTagHandler(Dart_LibraryTag tag,
   if (Dart_IsError(file_path)) {
     return file_path;
   }
-
-  Dart_Handle builtin_lib =
-      Builtin::LoadAndCheckLibrary(Builtin::kBuiltinLibrary);
-  Dart_Handle result = DartUtils::LoadSourceAsync(library,
-                                                  url,
-                                                  tag,
-                                                  builtin_lib);
-  if (Dart_IsError(result)) {
-    return result;
+  const char* raw_path = DartUtils::GetStringValue(file_path);
+  Dart_Handle source = DartUtils::ReadStringFromFile(raw_path);
+  if (Dart_IsError(source)) {
+    return source;
+  }
+  if (tag == Dart_kImportTag) {
+    return Dart_LoadLibrary(url, source, 0, 0);
+  } else {
+    ASSERT(tag == Dart_kSourceTag);
+    return Dart_LoadSource(library, url, source, 0, 0);
   }
-  return Dart_RunLoop();
 }