Make pub build use barback.

sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart

Index: sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
index ef5185ed6e78f93998f4d81a0ffdd3277723ff28..97d878a40b70f7c2a4d911a31a1e4e1f7f8606c9 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
@@ -26,6 +26,11 @@ import '../package_graph.dart';
 class Dart2JSTransformer extends Transformer {
   final PackageGraph _graph;
 
+  /// The [AssetId]s the transformer has discovered so far. Used by pub build
+  /// to determine where to copy the JS bootstrap files.
+  // TODO(rnystrom): Do something cleaner for this, or eliminate those files.

[nweiz, 2013/10/16 21:40:22]

A low-hanging way to do this would be to just put a browser.js file next to any
HTML file in web. There's already a TODO for this.

In the long term, though, we'll hopefully have a story for supporting assets in
general, and a way to refer to them in HTML. At that point we won't need any
explicit browser.js handling.

[Bob Nystrom, 2013/10/29 00:03:30]

Yeah, I talked to Vijay about this a bit while you were out and I think we'll
just eventually remove this stuff entirely once it isn't needed.

+  final entrypoints = new Set<AssetId>();
+
   Dart2JSTransformer(this._graph);
 
   /// Only ".dart" files within "web/" are processed.
@@ -60,6 +65,9 @@ class Dart2JSTransformer extends Transformer {
       // actually be on disk, but this gives dart2js a root to resolve
       // relative paths against.
       var id = transform.primaryInput.id;
+
+      entrypoints.add(id);
+
       var entrypoint = path.join(_graph.packages[id.package].dir, id.path);
       var packageRoot = path.join(_graph.entrypoint.root.dir, "packages");