Add a flag for building skyx packages

sky/tools/skyx/bin/skyx.dart

 // Copyright 2015 The Chromium Authors. 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';
 
 import 'package:archive/archive.dart';
 import 'package:args/args.dart';
 import 'package:yaml/yaml.dart';
@@ skipping 37 matching lines @@
       currentAssetDescriptor['theme'] = theme;
       for (String size in generateValues(assetDescriptor, 'size', kSizes)) {
         currentAssetDescriptor['size'] = size;
         yield new MaterialAsset(currentAssetDescriptor);
       }
     }
   }
 }
 
 Iterable<MaterialAsset> parseMaterialAssets(Map manifestDescriptor) sync* {
+  if (manifestDescriptor == null || !manifestDescriptor.containsKey('material-design-icons'))
+    return;
   for (Map assetDescriptor in manifestDescriptor['material-design-icons']) {
     for (MaterialAsset asset in generateMaterialAssets(assetDescriptor)) {
       yield asset;
     }
   }
 }
 
 Future loadManifest(String manifestPath) async {
   String manifestDescriptor = await new File(manifestPath).readAsString();
   return loadYaml(manifestDescriptor);
@@ skipping 34 matching lines @@
   String snapshot = args['snapshot'];
   if (snapshot != null)
     archive.addFile(await createSnapshotFile(snapshot));
 
   for (MaterialAsset asset in materialAssets)
     archive.addFile(await createFile(asset, args['asset-base']));
 
   File outputFile = new File(args['output-file']);
   await outputFile.writeAsBytes(new ZipEncoder().encode(archive));
 }