Fix a bug found by the analyzer

sky/sdk/lib/mojo/asset_bundle.dart

Index: sky/sdk/lib/mojo/asset_bundle.dart
diff --git a/sky/sdk/lib/mojo/asset_bundle.dart b/sky/sdk/lib/mojo/asset_bundle.dart
index 0a18bdd313b38527f4cbbee5c547082ba5d73457..7aa49c48977ce17a7ad626311d4ec14fbaf0ba6c 100644
--- a/sky/sdk/lib/mojo/asset_bundle.dart
+++ b/sky/sdk/lib/mojo/asset_bundle.dart
@@ -31,7 +31,7 @@ class NetworkAssetBundle extends AssetBundle {
 }
 
 Future _fetchAndUnpackBundle(String relativeUrl, AssetBundleProxy bundle) async {
-  core.MojoDataPipeConsumer bundleData = (await fetchUrl(url)).body;
+  core.MojoDataPipeConsumer bundleData = (await fetchUrl(relativeUrl)).body;
   AssetUnpackerProxy unpacker = new AssetUnpackerProxy.unbound();
   shell.requestService("mojo:asset_bundle", unpacker);
   unpacker.ptr.unpackZipStream(bundleData, bundle);