fix latest lints (dart-lang/source_maps#62)

pkgs/source_maps/lib/parser.dart

diff --git a/pkgs/source_maps/lib/parser.dart b/pkgs/source_maps/lib/parser.dart
index 1f8b817..99a3c81 100644
--- a/pkgs/source_maps/lib/parser.dart
+++ b/pkgs/source_maps/lib/parser.dart
@@ -531,7 +531,7 @@
 
     var url = urls[sourceUrlId];
     if (sourceRoot != null) {
-      url = '${sourceRoot}${url}';
+      url = '$sourceRoot$url';
     }
 
     var sourceNameId = entry.sourceNameId;