Add support for transformer clusters to barback.

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 3efce84417b0dfe2916411b21d39f0ceedce2e35..94ad902130bd337ae9268154e2dd834886b01ce2 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
@@ -52,7 +52,7 @@ Future loadAllTransformers(BarbackServer server, PackageGraph graph) {
   // "package:" imports while loading transformers.
   var rewrite = new RewriteImportTransformer();
   for (var package in graph.packages.values) {
-    server.barback.updateTransformers(package.name, [[rewrite]]);
+    server.barback.updateOperators(package.name, [[rewrite]]);
   }
 
   var orderingDeps = _computeOrderingDeps(graph);
@@ -90,11 +90,11 @@ Future loadAllTransformers(BarbackServer server, PackageGraph graph) {
             .map(loader.load)).then((_) {
           transformers.add(unionAll(phase.map(
               (id) => loader.transformersFor(id))));
-          server.barback.updateTransformers(package, transformers);
+          server.barback.updateOperators(package, transformers);
         });
       }).then((_) {
         // Now that we've applied all the transformers used by [package] via
-        // [Barback.updateTransformers], we load any transformers defined in
+        // [Barback.updateOperators], we load any transformers defined in
         // [package] but used elsewhere.
         return Future.wait(packageTransformers[package].map(loader.load));
       });
@@ -110,7 +110,7 @@ Future loadAllTransformers(BarbackServer server, PackageGraph graph) {
       var phases = package.pubspec.transformers.map((phase) {
         return unionAll(phase.map((id) => loader.transformersFor(id)));
       });
-      server.barback.updateTransformers(package.name, phases);
+      server.barback.updateOperators(package.name, phases);
     }
   });
 }