Fix URL/native path conversions in pub.

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 3c942a20b28369d78fa2433149da350698b6be48..6f21c8de5360dab40b6749273d90590a73227703 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
@@ -292,7 +292,8 @@ class _BarbackCompilerProvider implements dart.CompilerProvider {
     if (name == "") {
       outPath = _transform.primaryInput.id.path;
     } else {
-      outPath = path.join(path.dirname(_transform.primaryInput.id.path), name);
+      var dirname = path.url.dirname(_transform.primaryInput.id.path);
+      outPath = path.url.join(dirname, name);
     }
 
     var id = new AssetId(primaryId.package, "$outPath.$extension");
@@ -393,8 +394,8 @@ class _BarbackCompilerProvider implements dart.CompilerProvider {
     // should be loaded directly from disk.
     var sourcePath = path.fromUri(url);
     if (_environment.containsPath(sourcePath)) {
-      var relative = path.relative(sourcePath,
-          from: _environment.rootPackage.dir);
+      var relative = path.toUri(path.relative(sourcePath,
+          from: _environment.rootPackage.dir)).toString();
 
       return new AssetId(_environment.rootPackage.name, relative);
     }