Allow mapping of builtin libraries 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 317 matching lines @@
       return url;
     }
     // Parts of internal libraries are handled internally.
     if (libraryBuiltinId != Builtin::kInvalidLibrary) {
       return url;
     }
     return ResolveUri(library_url_string, url_string);
   }
 
   Builtin::BuiltinLibraryId builtinId = BuiltinId(url_string);
-  if (builtinId != Builtin::kInvalidLibrary) {
-    // Special case for importing a builtin library.
+  if ((builtinId != Builtin::kInvalidLibrary) && (mapped_url_string == NULL)) {
+    // Special case for importing a builtin library that isn't remapped.
     if (tag == Dart_kImportTag) {
       return Builtin::LoadLibrary(url, builtinId);
     }
     ASSERT(tag == Dart_kSourceTag);
     return DartUtils::NewError("Unable to part '%s' ", url_string);
   }
 
   if (libraryBuiltinId != Builtin::kInvalidLibrary) {
     // Special case for parting sources of a builtin library.
     if (tag == Dart_kSourceTag) {
@@ skipping 280 matching lines @@
   }
   return 0;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   return dart::bin::main(argc, argv);
 }