make dart2js_info strong-mode clean

bin/inference/print_summary.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.
 
 /// Utility to display statistics about "sends" as a table on the command line.
 library compiler.tool.stats.print_summary;
 
 import 'dart:io';
 import 'dart:convert';
 import 'package:dart2js_info/info.dart';
@@ skipping 63 matching lines @@
     var uri = info.uri;
     var bundle = uri.scheme == 'package'
         ? uri.pathSegments.first
         : uri.scheme == 'file' ? uri.pathSegments.last : '$uri';
     currentBundleTotals =
         bundleTotals.putIfAbsent(bundle, () => new Measurements());
     super.visitLibrary(info);
     totals.addFrom(currentBundleTotals);
   }
 
-  visitFunction(FunctionInfo function) {
+  Null visitFunction(FunctionInfo function) {
     var measurements = function.measurements;
-    if (measurements == null) return;
+    if (measurements == null) return null;
     currentBundleTotals.addFrom(measurements);
+    return null;
   }
 }
 
 const _groupColors = const [_YELLOW_COLOR, _NO_COLOR];
 
 const _NO_COLOR = "\x1b[0m";
 const _GREEN_COLOR = "\x1b[32m";
 const _YELLOW_COLOR = "\x1b[33m";
 const _WHITE_COLOR = "\x1b[37m";