Don't limit transformer parallelism 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';
@@ skipping 51 matching lines @@
 
   /// 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;
 
   /// The stack trace for [_lastUnexpectedError].
   StackTrace _lastUnexpectedErrorTrace;
 
-  // 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 transformPool = new Pool(10);
-
   /// Creates a new [PackageGraph] that will transform assets in all packages
   /// made available by [provider].
   PackageGraph(this.provider) {
     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;
       cascade.onDirty.listen((_) {
@@ skipping 103 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);
   }
 }