Don't use async loading in gen_snapshot.

runtime/bin/gen_snapshot.cc

 // Copyright (c) 2013, 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.
 
 // Generate a snapshot file after loading all the scripts specified on the
 // command line.
 
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
@@ skipping 333 matching lines @@
       return resolved_url;
     }
   }
 
   // Get the file path out of the url.
   Dart_Handle file_path = FilePathFromUri(
       DartUtils::GetStringValue(resolved_url));
   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;
   }
-  return Dart_RunLoop();
+  if (tag == Dart_kImportTag) {
+    return Dart_LoadLibrary(url, source, 0, 0);
+  } else {
+    ASSERT(tag == Dart_kSourceTag);
+    return Dart_LoadSource(library, url, source, 0, 0);
+  }
 }
 
 
 static Dart_Handle LoadSnapshotCreationScript(const char* script_name) {
   Dart_Handle resolved_script_uri = ResolveScriptUri(script_name);
   if (Dart_IsError(resolved_script_uri)) {
     return resolved_script_uri;
   }
   Dart_Handle script_path = FilePathFromUri(
       DartUtils::GetStringValue(resolved_script_uri));
@@ skipping 214 matching lines @@
   }
   return 0;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   return dart::bin::main(argc, argv);
 }