Add a flag for building skyx packages

sky/tools/skyx/bin/skyx.dart

Index: sky/tools/skyx/bin/skyx.dart
diff --git a/sky/tools/skyx/bin/skyx.dart b/sky/tools/skyx/bin/skyx.dart
index 1bc27b8aa19594045c90fe207e6cde9e15ca0edb..989b5c5d204957fcfdb922741df7ce0d93a6b64c 100644
--- a/sky/tools/skyx/bin/skyx.dart
+++ b/sky/tools/skyx/bin/skyx.dart
@@ -55,6 +55,8 @@ Iterable<MaterialAsset> generateMaterialAssets(Map assetDescriptor) sync* {
 }
 
 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;