Only run at most 10 transformers at once in barback.

pkg/barback/lib/src/package_graph.dart

Index: pkg/barback/lib/src/package_graph.dart
diff --git a/pkg/barback/lib/src/package_graph.dart b/pkg/barback/lib/src/package_graph.dart
index 68753f652f88cfd960caf244ec032cb59c8a178c..ad2a9290e71738a3195b323bf197e064a2ed6459 100644
--- a/pkg/barback/lib/src/package_graph.dart
+++ b/pkg/barback/lib/src/package_graph.dart
@@ -14,6 +14,7 @@ import 'barback_logger.dart';
 import 'build_result.dart';
 import 'errors.dart';
 import 'package_provider.dart';
+import 'pool.dart';
 import 'transformer.dart';
 import 'utils.dart';
 
@@ -65,6 +66,13 @@ class PackageGraph {
   /// [Future] returned by [getAllAssets].
   var _lastUnexpectedError;
 
+  // TODO(nweiz): Allow transformers to declare themselves as "lightweight" or
+  // "heavyweight" and adjust their restrictions appropriately. Simple
+  // transformers may be very efficient to run in parallel, whereas dart2js uses
+  // a lot of memory and should be run more sequentially.
+  /// A pool that controls how many transformers may be applied at once.
+  final Pool transformerPool = new Pool(10);
+
   /// Creates a new [PackageGraph] that will transform assets in all packages
   /// made available by [provider].
   PackageGraph(this.provider, {BarbackLogger logger})