Precompile immutable dependencies on "pub get".

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

Index: sdk/lib/_internal/pub/lib/src/barback/load_all_transformers.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/load_all_transformers.dart b/sdk/lib/_internal/pub/lib/src/barback/load_all_transformers.dart
index 3b156de5fccd80ab632e8773fbb69809541d5dd1..bba80c574d2cb05459abe9747fd50e3d0b747598 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_all_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_all_transformers.dart
@@ -94,7 +94,8 @@ Future loadAllTransformers(AssetEnvironment environment,
 
   /// Reset the transformers for each package to get rid of [rewrite], which
   /// is no longer needed.
-  await Future.wait(environment.graph.packages.values.map((package) async {
+  await Future.wait(environment.packages.map((packageName) async {
+    var package = environment.graph.packages[packageName];
     var phases = await loader.transformersForPhases(
         package.pubspec.transformers);
     var transformers = environment.getBuiltInTransformers(package);