Add fallback to load the original version of dart:io library

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 278 matching lines @@
                                                            url_string);
 
   if (tag == kCanonicalizeUrl) {
     // Keep original names for libraries that have a mapping (e.g. dart:io).
     if (mapped_url_string) {
       return url;
     }
     return ResolveUri(library_url_string, url_string);
   }
 
+  if (DartUtils::IsDartIOLibURL(url_string) && mapped_url_string == NULL) {
+    // No url mapping for dart:io. Load original version.
+    return Builtin::LoadAndCheckLibrary(Builtin::kIOLibrary);
+  }
+
   Dart_Handle resolved_url = url;
   if (mapped_url_string != NULL) {
     // Mapped urls are relative to working directory.
     resolved_url = ResolveScriptUri(mapped_url_string);
     if (Dart_IsError(resolved_url)) {
       return resolved_url;
     }
   }
 
   // Get the file path out of the url.
@@ skipping 212 matching lines @@
     CreateAndWriteSnapshot();
 
     Dart_EnterIsolate(UriResolverIsolateScope::isolate);
     Dart_ShutdownIsolate();
   } else {
     SetupForGenericSnapshotCreation();
     CreateAndWriteSnapshot();
   }
   return 0;
 }