Fix for default filter (#228).

bin/linter.dart

Index: bin/linter.dart
diff --git a/bin/linter.dart b/bin/linter.dart
index 45af03027081b13397ab6baef01a2c28af6102e7..5c5670bf30c39def79b66fbd5de2b670d733e33e 100644
--- a/bin/linter.dart
+++ b/bin/linter.dart
@@ -174,12 +174,16 @@ $stack''');
   }
 }
 
+Iterable<AnalysisError> _filtered(
+        List<AnalysisError> errors, LintFilter filter) =>
+    (filter == null)
+        ? errors
+        : errors.where((AnalysisError e) => !filter.filter(e));
+
 int _maxSeverity(List<AnalysisErrorInfo> errors, LintFilter filter) {
   int max = 0;
   for (AnalysisErrorInfo info in errors) {
-    info.errors
-        .where((AnalysisError e) => !filter.filter(e))
-        .forEach((AnalysisError e) {
+    _filtered(info.errors, filter).forEach((AnalysisError e) {
       max = math.max(max, e.errorCode.errorSeverity.ordinal);
     });
   }