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

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

Index: sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart b/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
index 1591046753bae989f27f3ba6f97266ec0955c298..6f2f09afedc86d10069a5e8a7016ace84fc66a74 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
@@ -56,11 +56,16 @@ class AssetEnvironment {
   /// 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;
@@ -74,7 +79,7 @@ class AssetEnvironment {
       var environment = new AssetEnvironment._(graph, barback, mode,
           watcherType, hostname, basePort);
 
-      return environment._load(useDart2JS: useDart2JS)
+      return environment._load(entrypoints: entrypoints, useDart2JS: useDart2JS)
           .then((_) => environment);
     });
   }
@@ -445,9 +450,12 @@ class AssetEnvironment {
   /// 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.
@@ -504,7 +512,8 @@ class AssetEnvironment {
 
         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]);