dart2js: fix analyzer warning

pkg/compiler/lib/src/stats/builder.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.
 
 /// Computes statistical data about a program. See `stats.dart` for more details
 /// on the kind of data we collect.
 library compiler.src.stats.stats_builder;
 
 import 'dart:convert';
 import 'package:dart2js_info/src/measurements.dart';
@@ skipping 18 matching lines @@
 
 import 'analysis_result.dart';
 import 'naive_analysis_result.dart';
 import 'trusted_types_analysis_result.dart';
 
 /// Collects a set of [Measurements] about send expressions in the function [f].
 // TODO(sigmund): collect information on initializers too.
 Measurements collectSendMeasurements(FunctionElement f, Compiler compiler) {
   return compiler.withCurrentElement(f, () {
     // TODO(sigmund): enable for platform too.
-    if (f.library.isPlatformLibrary) return;
+    if (f.library.isPlatformLibrary) return null;
     var name = _qualifiedName(f);
     if (!f.hasNode) {
       if (f is PartialElement) return const Measurements.unreachableFunction();
       assert (f is ConstructorElement && f.isSynthesized);
       // TODO(sigmund): measure synthethic forwarding sends, measure
       // initializers
       return new Measurements.reachableFunction();
     }
     if (!f.hasResolvedAst) {
       _debug('no resolved ast ${f.runtimeType}');
@@ skipping 2322 matching lines @@
   @override
   R visitClosureFieldElement(ClosureFieldElement e, A arg) {
     return visitVariableElement(e, arg);
   }
 }
 
 // TODO(sigmund): get rid of debug messages.
 _debug(String message) {
   print('debug: $message');
 }