switch package to strong mode

lib/assets.dart

Index: lib/assets.dart
diff --git a/lib/assets.dart b/lib/assets.dart
index 4a3516f63be475bf4314fc8ea7fa79228c131f5a..921b824743d121836845acf486a55fcdd322cde2 100644
--- a/lib/assets.dart
+++ b/lib/assets.dart
@@ -83,7 +83,7 @@ AssetId uriToAssetId(
       return _extractOtherPackageId(1, segments, logger, span);
     } else {
       var prefix = segments[index];
-      var fixedSegments = [];
+      var fixedSegments = <String>[];
       fixedSegments.addAll(sourceSegments.map((_) => '..'));
       fixedSegments.addAll(segments.sublist(index));
       var fixedUrl = urlBuilder.joinAll(fixedSegments);
@@ -99,7 +99,7 @@ AssetId uriToAssetId(
 }
 
 AssetId _extractOtherPackageId(
-    int index, List segments, TransformLogger logger, SourceSpan span) {
+    int index, List<String> segments, TransformLogger logger, SourceSpan span) {
   if (index >= segments.length) return null;
   var prefix = segments[index];
   if (prefix != 'packages' && prefix != 'assets') return null;