Don't use async loading in gen_snapshot.

runtime/bin/dartutils.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef BIN_DARTUTILS_H_
 #define BIN_DARTUTILS_H_
 
 #include "include/dart_api.h"
 #include "include/dart_native_api.h"
 
@@ skipping 106 matching lines @@
   static void ReadFile(const uint8_t** data, intptr_t* file_len, void* stream);
   static void WriteFile(const void* buffer, intptr_t num_bytes, void* stream);
   static void CloseFile(void* stream);
   static bool EntropySource(uint8_t* buffer, intptr_t length);
   static Dart_Handle ReadStringFromFile(const char* filename);
   static Dart_Handle LibraryTagHandler(Dart_LibraryTag tag,
                                        Dart_Handle library,
                                        Dart_Handle url);
   static Dart_Handle LoadScript(const char* script_uri,
                                 Dart_Handle builtin_lib);
-  static Dart_Handle LoadSourceAsync(Dart_Handle library,
-                                     Dart_Handle url,
-                                     Dart_LibraryTag tag,
-                                     Dart_Handle builtin_lib);
   static Dart_Handle PrepareForScriptLoading(const char* package_root,
                                              Dart_Handle builtin_lib);
 
   static bool PostNull(Dart_Port port_id);
   static bool PostInt32(Dart_Port port_id, int32_t value);
   static bool PostInt64(Dart_Port port_id, int64_t value);
 
   static Dart_Handle GetDartType(const char* library_url,
                                  const char* class_name);
   // Create a new Dart OSError object with the current OS error.
@@ skipping 403 matching lines @@
 
   ~ScopedBlockingCall() {
     Dart_IsolateUnblocked();
   }
 };
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // BIN_DARTUTILS_H_