Allow [path.fromUri] to take a string as well as a URI.

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

Index: sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart b/sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart
index 7740ba60f450c4c165ecdc4880b04a984a2f27bc..dd90b9f688a947de75b58271af16c86126708d41 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart
@@ -24,7 +24,7 @@ class PubPackageProvider implements PackageProvider {
 
   Future<Asset> getAsset(AssetId id) {
     if (id.package != r'$pub') {
-      var nativePath = path.joinAll(path.url.split(id.path));
+      var nativePath = path.fromUri(id.path);
       var file = path.join(_graph.packages[id.package].dir, nativePath);
       return new Future.value(new Asset.fromPath(id, file));
     }