Fix bug in native/behavior.dart

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

Index: pkg/compiler/lib/src/native/behavior.dart
diff --git a/pkg/compiler/lib/src/native/behavior.dart b/pkg/compiler/lib/src/native/behavior.dart
index f1a011e763a3491f9a0caa26bb87ae654d1925e8..814846ce5a658700a1c623b5327c5482d3a43ac1 100644
--- a/pkg/compiler/lib/src/native/behavior.dart
+++ b/pkg/compiler/lib/src/native/behavior.dart
@@ -221,7 +221,8 @@ class NativeBehavior {
         .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>{};