make dart2js_info strong-mode clean

bin/inference/client.dart

Index: bin/inference/client.dart
diff --git a/bin/inference/client.dart b/bin/inference/client.dart
index e5b05d79728629405d104ea7eacc18aaf0542ba0..caba830fd53d6cc10418b25e79e5f63fbca446f1 100644
--- a/bin/inference/client.dart
+++ b/bin/inference/client.dart
@@ -116,8 +116,8 @@ class SendHighlighter extends RecursiveInfoVisitor {
   }
 
   @override
-  visitFunction(FunctionInfo function) {
-    if (function.measurements?.uri?.path != path) return;
+  Null visitFunction(FunctionInfo function) {
+    if (function.measurements?.uri?.path != path) return null;
     var entries = function.measurements.entries;
     for (var metric in entries.keys) {
       if (metric is GroupedMetric) continue;
@@ -128,6 +128,7 @@ class SendHighlighter extends RecursiveInfoVisitor {
         code.insert(entry.end, '</span>');
       }
     }
+    return null;
   }
 }