make dart2js_info strong-mode clean

bin/inference/client.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Client component to display [GlobalResult]s as a web app.
 library dart2js_info.bin.inference.client;
 
 import 'dart:html' hide Entry;
 import 'dart:convert';
 import 'package:dart2js_info/info.dart';
@@ skipping 98 matching lines @@
       } else if (contents.codeUnitAt(i) == $gt) {
         code.replace(i, i + 1, '>');
       } else if (contents.codeUnitAt(i) == $lf) {
         code.insert(i + 1, '</span><span class="line">');
       }
     }
     code.insert(contents.length, '</span>');
   }
 
   @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;
       var cssClassName = _classNameForMetric(metric);
       for (var entry in entries[metric]) {
         code.insert(entry.begin, '<span class="send ${cssClassName} inactive">',
             -entry.end);
         code.insert(entry.end, '</span>');
       }
     }
+    return null;
   }
 }
 
 _classNameForMetric(Metric metric) => metric.name.replaceAll(' ', '-');