Pass a package root to spawned isolates in pub.

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 bba80c574d2cb05459abe9747fd50e3d0b747598..9ba4aa690480eb15f6cb58a0158d2b3a11116d2d 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
@@ -11,8 +11,6 @@ import '../package_graph.dart';
 import '../utils.dart';
 import 'asset_environment.dart';
 import 'barback_server.dart';
-import 'rewrite_import_transformer.dart';
-import 'transformer_cache.dart';
 import 'transformer_id.dart';
 import 'transformer_loader.dart';
 import 'transformers_needed_by_transformers.dart';
@@ -49,14 +47,6 @@ Future loadAllTransformers(AssetEnvironment environment,
 
   var loader = new TransformerLoader(environment, transformerServer);
 
-  // Add a rewrite transformer for each package, so that we can resolve
-  // "package:" imports while loading transformers.
-  var rewrite = new RewriteImportTransformer();
-  for (var package in environment.packages) {
-    environment.barback.updateTransformers(package, [[rewrite]]);
-  }
-  environment.barback.updateTransformers(r'$pub', [[rewrite]]);
-
   // Only save compiled snapshots when a physical entrypoint package is being
   // used. There's no physical entrypoint when e.g. globally activating a cached
   // package.
@@ -82,24 +72,21 @@ Future loadAllTransformers(AssetEnvironment environment,
       var package = environment.graph.packages[packageName];
       var phases = await loader.transformersForPhases(
           package.pubspec.transformers);
-
-      // Make sure [rewrite] is still the first phase so that future
-      // transformers' "package:" imports will work.
-      phases.insert(0, new Set.from([rewrite]));
       environment.barback.updateTransformers(packageName, phases);
     }));
   }
 
   if (cache != null) cache.save();
 
-  /// Reset the transformers for each package to get rid of [rewrite], which
-  /// is no longer needed.
+  /// Add built-in transformers for the packages that need them.
   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);
-    if (transformers != null) phases.add(transformers);
+    if (transformers == null) return;
+
+    phases.add(transformers);
 
     // TODO(nweiz): remove the [newFuture] here when issue 17305 is fixed.
     // If no transformer in [phases] applies to a source input,