Fix for default filter (#228).

bin/linter.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.
 
 import 'dart:io';
 import 'dart:math' as math;
 
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/error.dart';
 import 'package:args/args.dart';
@@ skipping 156 matching lines @@
         quiet: options['quiet']);
     reporter.write();
   } catch (err, stack) {
     errorSink.writeln('''An error occurred while linting
   Please report it at: github.com/dart-lang/linter/issues
 $err
 $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);
     });
   }
   return max;
 }