Fix dart2js transformer on Windows.

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 ec59537d610792a4d300d59e9ad5897e5226345e..9405f43d4c3d56f9e8a574deba665ca0158bdb7a 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
@@ -53,10 +53,7 @@ class Dart2JSTransformer extends Transformer {
       // actually be on disk, but this gives dart2js a root to resolve
       // relative paths against.
       var id = transform.primaryInput.id;
-      var entrypoint = path.url.join(
-          path.toUri(_graph.packages[id.package].dir).path,
-          id.path);
-
+      var entrypoint = path.join(_graph.packages[id.package].dir, id.path);
       var packageRoot = path.join(_graph.entrypoint.root.dir, "packages");
 
       // TODO(rnystrom): Should have more sophisticated error-handling here.