Fix bug in native/behavior.dart

pkg/compiler/lib/src/native/behavior.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.
 
 part of native;
 
 /// This class is a temporary work-around until we get a more powerful DartType.
 class SpecialType {
   final String name;
   const SpecialType._(this.name);
@@ skipping 203 matching lines @@
         typesReturned.add(type);
       });
       return;
     }
 
     List<String> specs = specString.split(';')
         .map((s) => s.trim())
         .toList();
     if (specs.last == "") specs.removeLast();  // Allow separator to terminate.
 
-    assert(validTags == null || validTags.toSet().removeAll(validTags).isEmpty);
+    assert(validTags == null ||
+           (validTags.toSet()..removeAll(validTags)).isEmpty);
     if (validTags == null) validTags = knownTags;
 
     Map<String, String> values = <String, String>{};
 
     for (String spec in specs) {
       List<String> tagAndValue = spec.split(':');
       if (tagAndValue.length != 2) {
         listener.internalError(spannable,
             "Invalid <tag>:<value> pair '$spec'.");
       }
@@ skipping 447 matching lines @@
         _errorNode(locationNodeOrElement, compiler),
         "Type '$typeString' not found.");
     return null;
   }
 
   static _errorNode(locationNodeOrElement, compiler) {
     if (locationNodeOrElement is Node) return locationNodeOrElement;
     return locationNodeOrElement.parseNode(compiler);
   }
 }