Remove AsyncError with Expando.

sdk/lib/_internal/dartdoc/bin/dartdoc.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.
 
 /**
  * To generate docs for a library, run this script with the path to an
  * entrypoint .dart file, like:
  *
  *     $ dart dartdoc.dart foo.dart
  *
@@ skipping 233 matching lines @@
     .then((_) => copyDirectory(scriptDir.append('../static'),
           dartdoc.outputDir))
     .then((_) {
       print(dartdoc.status);
       if (dartdoc.totals == 0) {
         exit(1);
       }
     })
     .catchError((e) {
       print('Error: generation failed: ${e}');
+      var trace = getAttachedStackTrace(e);
+      if (trace != null) print("StackTrace: $trace");
       dartdoc.cleanup();
       exit(1);
     })
     .whenComplete(() => dartdoc.cleanup());
 }
 
 String _getPackageRoot(List<Uri> entrypoints) {
   // Check if there's a `packages` directory in the entry point directory.
   var fileEntrypoint = entrypoints.firstWhere(
       (entrypoint) => entrypoint.scheme == 'file',
       orElse: () => null);
   if (fileEntrypoint == null) return;
 
   var script = path.normalize(path.absolute(fileUriToPath(fileEntrypoint)));
   var dir = path.join(path.dirname(script), 'packages/');
   if (new Directory(dir).existsSync()) return dir;
 
   // If there is not, then check if the entrypoint is somewhere in a `lib`
   // directory.
   var parts = path.split(path.dirname(script));
   var libDir = parts.lastIndexOf('lib');
   if (libDir > 0) {
     return path.join(path.joinAll(parts.take(libDir)), 'packages');
   } else {
     return null;
   }
 }