Use precompiled dependencies in pub.

sdk/lib/_internal/pub/lib/src/barback/transformer_loader.dart

Index: sdk/lib/_internal/pub/lib/src/barback/transformer_loader.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/transformer_loader.dart b/sdk/lib/_internal/pub/lib/src/barback/transformer_loader.dart
index 2d9f3338095784b51bb0ec817db11e3873835f9a..92a8ed9494c85782c52502aa956a048983ececaa 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/transformer_loader.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/transformer_loader.dart
@@ -35,7 +35,8 @@ class TransformerLoader {
 
   TransformerLoader(this._environment, this._transformerServer) {
     for (var package in _environment.graph.packages.values) {
-      for (var config in unionAll(package.pubspec.transformers)) {
+      var transformers = unionAll(package.pubspec.transformers);
+      for (var config in transformers) {
         _transformerUsers.putIfAbsent(config.id, () => new Set<String>())
             .add(package.name);
       }