Log only valid occurences of 'All native types used ...'.

pkg/compiler/lib/src/native/enqueue.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:collection' show Queue;
 
 import '../common.dart';
 import '../common/backend_api.dart' show ForeignResolver;
 import '../common/resolution.dart' show Resolution;
 import '../compiler.dart' show Compiler;
@@ skipping 469 matching lines @@
         assert(type is VoidType);
       }
     }
     if (matchingClasses.isNotEmpty && _registeredClasses.isEmpty) {
       matchingClasses.addAll(_onFirstNativeClass(impactBuilder));
     }
     _registerTypeUses(impactBuilder, matchingClasses, cause);
 
     // Give an info so that library developers can compile with -v to find why
     // all the native classes are included.
-    if (unusedBefore == matchingClasses.length) {
+    if (unusedBefore > 0 && unusedBefore == matchingClasses.length) {
       reporter.log('All native types marked as used due to $cause.');
     }
   }
 
   Iterable<ClassElement> _findUnusedClassesMatching(
       bool predicate(classElement)) {
     return _unusedClasses.where(predicate);
   }
 
   Iterable<ClassElement> _onFirstNativeClass(WorldImpactBuilder impactBuilder) {
@@ skipping 161 matching lines @@
     List<Element> directSubtypes =
         emitter.directSubtypes.putIfAbsent(superclass, () => <ClassElement>[]);
     directSubtypes.add(cls);
   }
 
   void logSummary(log(message)) {
     log('Compiled ${_registeredClasses.length} native classes, '
         '${_unusedClasses.length} native classes omitted.');
   }
 }