Only run at most 10 transformers at once in barback.

pkg/barback/lib/src/package_graph.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library barback.package_graph;
 
 import 'dart:async';
 
 import 'asset_cascade.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
 import 'asset_set.dart';
 import 'barback_logger.dart';
 import 'build_result.dart';
 import 'errors.dart';
 import 'package_provider.dart';
+import 'pool.dart';
 import 'transformer.dart';
 import 'utils.dart';
 
 /// The collection of [AssetCascade]s for an entire application.
 ///
 /// This tracks each package's [AssetCascade] and routes asset requests between
 /// them.
 class PackageGraph {
   /// The provider that exposes asset and package information.
   final PackageProvider provider;
@@ skipping 31 matching lines @@
   /// emitted through the [results] stream's error channel.
   Stream<BarbackException> get errors => _errors;
   Stream<BarbackException> _errors;
 
   /// The most recent error emitted from a cascade's result stream.
   ///
   /// This is used to pipe an unexpected error from a build to the resulting
   /// [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})
       : _logger = logger != null ? logger : new BarbackLogger() {
     for (var package in provider.packages) {
       var cascade = new AssetCascade(this, package);
       // The initial result for each cascade is "success" since the cascade
       // doesn't start building until some source in that graph is updated.
       _cascadeResults[package] = new BuildResult.success();
       _cascades[package] = cascade;
@@ skipping 87 matching lines @@
       if (cascade == null) throw new ArgumentError("Unknown package $package.");
       cascade.removeSources(ids);
     });
   }
 
   void updateTransformers(String package,
       Iterable<Iterable<Transformer>> transformers) {
     _cascades[package].updateTransformers(transformers);
   }
 }