Remove AsyncError with Expando.

tools/version.dart

 #!/usr/bin/env dart
 // 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.
 
 import "dart:io";
+import "dart:async" show getAttachedStackTrace;
 import "release/version.dart";
 
 void main() {
   Path scriptPath = new Path(new Options().script).directoryPath;
   Version version = new Version(scriptPath.append("VERSION"));
   Future f = version.getVersion();
   f.then((currentVersion) {
     print(currentVersion);
   }).catchError((e) {
     print("Could not create version number, failed with: $e");
+    var trace = getAttachedStackTrace(e);
+    if (trace != null) print("StackTrace: $trace");
     return true;
   });
 }