Don't load transformers that aren't going to be used for an executable.

sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart

 // Copyright (c) 2014, 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 pub.barback.asset_environment;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:barback/barback.dart';
@@ skipping 38 matching lines @@
   ///
   /// This will only add the root package's "lib" directory to the environment.
   /// Other directories can be added to the environment using [serveDirectory].
   ///
   /// If [watcherType] is not [WatcherType.NONE] (the default), watches source
   /// assets for modification.
   ///
   /// If [packages] is passed, only those packages' assets will be loaded and
   /// served.
   ///
+  /// If [entrypoints] is passed, only transformers necessary to run those
+  /// entrypoints will be loaded. Each entrypoint is expected to refer to a Dart

[Bob Nystrom, 2014/09/24 23:42:47]

"will be" -> "are".

It took me a while to break my future tense habit, but I'm glad I did.

[nweiz, 2014/09/25 00:13:15]

Done.

+  /// library.
+  ///
   /// Returns a [Future] that completes to the environment once the inputs,
   /// transformers, and server are loaded and ready.
   static Future<AssetEnvironment> create(Entrypoint entrypoint,
       BarbackMode mode, {WatcherType watcherType, String hostname, int basePort,
-      Iterable<String> packages, bool useDart2JS: true}) {
+      Iterable<String> packages, Iterable<AssetId> entrypoints,
+      bool useDart2JS: true}) {
     if (watcherType == null) watcherType = WatcherType.NONE;
     if (hostname == null) hostname = "localhost";
     if (basePort == null) basePort = 0;
 
     return entrypoint.loadPackageGraph().then((graph) {
       log.fine("Loaded package graph.");
       graph = _adjustPackageGraph(graph, mode, packages);
       var barback = new Barback(new PubPackageProvider(graph));
       barback.log.listen(_log);
 
       var environment = new AssetEnvironment._(graph, barback, mode,
           watcherType, hostname, basePort);
 
-      return environment._load(useDart2JS: useDart2JS)
+      return environment._load(entrypoints: entrypoints, useDart2JS: useDart2JS)
           .then((_) => environment);
     });
   }
 
   /// Return a version of [graph] that's restricted to [packages] (if passed)
   /// and loads cached packages (if [mode] is [BarbackMode.DEBUG]).
   static PackageGraph _adjustPackageGraph(PackageGraph graph,
       BarbackMode mode, Iterable<String> packages) {
     if (mode != BarbackMode.DEBUG && packages == null) return graph;
     packages = (packages == null ? graph.packages.keys : packages).toSet();
@@ skipping 350 matching lines @@
   /// Loads the assets and transformers for this environment.
   ///
   /// This transforms and serves all library and asset files in all packages in
   /// the environment's package graph. It loads any transformer plugins defined
   /// in packages in [graph] and re-runs them as necessary when any input files
   /// change.
   ///
   /// If [useDart2JS] is `true`, then the [Dart2JSTransformer] is implicitly
   /// added to end of the root package's transformer phases.
   ///
+  /// If [entrypoints] is passed, only transformers necessary to run those
+  /// entrypoints will be loaded.
+  ///
   /// Returns a [Future] that completes once all inputs and transformers are
   /// loaded.
-  Future _load({bool useDart2JS}) {
+  Future _load({Iterable<AssetId> entrypoints, bool useDart2JS}) {
     return log.progress("Initializing barback", () {
       // If the entrypoint package manually configures the dart2js
       // transformer, don't include it in the built-in transformer list.
       //
       // TODO(nweiz): if/when we support more built-in transformers, make
       // this more general.
       var containsDart2JS = graph.entrypoint.root.pubspec.transformers
           .any((transformers) =>
               transformers.any((config) => config.id.package == '\$dart2js'));
 
@@ skipping 36 matching lines @@
           if (error.stackTrace != null) {
             message += "\n" + error.stackTrace.terse.toString();
           }
 
           _log(new LogEntry(error.transform, error.transform.primaryId,
                   LogLevel.ERROR, message, null));
         });
 
         return _withStreamErrors(() {
           return log.progress("Loading transformers", () {
-            return loadAllTransformers(this, transformerServer)
+            return loadAllTransformers(this, transformerServer,
+                    entrypoints: entrypoints)
                 .then((_) => transformerServer.close());

[Bob Nystrom, 2014/09/24 23:42:47]

Whole lotta indentation here. How about using await?

[nweiz, 2014/09/25 00:13:15]

Done.

           }, fine: true);
         }, [errorStream, barback.results, transformerServer.results]);
       });
     }, fine: true);
   }
 
   /// Provides the public source assets in the environment to barback.
   ///
   /// If [watcherType] is not [WatcherType.NONE], enables watching on them.
   Future _provideSources() async {
@@ skipping 264 matching lines @@
   String toString() => "polling";
 }
 
 class _NoneWatcherType implements WatcherType {
   const _NoneWatcherType();
 
   DirectoryWatcher create(String directory) => null;
 
   String toString() => "none";
 }