(TBR) fix PubPackageMapProvider to use resourceProvider context for path

pkg/analyzer/lib/source/pub_package_map_provider.dart

Index: pkg/analyzer/lib/source/pub_package_map_provider.dart
diff --git a/pkg/analyzer/lib/source/pub_package_map_provider.dart b/pkg/analyzer/lib/source/pub_package_map_provider.dart
index b9b19c677589b2885872302fd0f70f213bbe4e18..f0306411124d2aafcdb15a1102487d782873f0ac 100644
--- a/pkg/analyzer/lib/source/pub_package_map_provider.dart
+++ b/pkg/analyzer/lib/source/pub_package_map_provider.dart
@@ -96,7 +96,8 @@ class PubPackageMapProvider implements PackageMapProvider {
     // we'll know when to try running "pub list-package-dirs" again.
     // Unfortunately, "pub list-package-dirs" doesn't tell us dependencies when
     // an error occurs, so just assume there is one dependency, "pubspec.lock".
-    List<String> dependencies = <String>[join(folder.path, PUBSPEC_LOCK_NAME)];
+    List<String> dependencies = <String>[
+        resourceProvider.pathContext.join(folder.path, PUBSPEC_LOCK_NAME)];
     return new PackageMapInfo(null, dependencies.toSet());
   }