Don't implicitly run "pub get".

lib/src/barback/asset_environment.dart

Index: lib/src/barback/asset_environment.dart
diff --git a/lib/src/barback/asset_environment.dart b/lib/src/barback/asset_environment.dart
index a8a56f0df048f8eb2b6c84f1ae24643a9ddb241e..657d5a58b0d11d8ca2dddde790cbb23ce2213589 100644
--- a/lib/src/barback/asset_environment.dart
+++ b/lib/src/barback/asset_environment.dart
@@ -74,8 +74,7 @@ class AssetEnvironment {
     if (environmentConstants == null) environmentConstants = {};
 
     return log.progress("Loading asset environment", () async {
-      var graph = await entrypoint.loadPackageGraph();
-      graph = _adjustPackageGraph(graph, mode, packages);
+      var graph = _adjustPackageGraph(entrypoint.packageGraph, mode, packages);
       var barback = new Barback(new PubPackageProvider(graph));
       barback.log.listen(_log);