Fix for default filter (#228).

The rub is that when I added awareness of filters to max error calculation, I forgot to account for the case where there was none set.  This should address that, with a test added.

BUG=
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Review URL: https://codereview.chromium.org//1918933002 .

Author: pq

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);
     });
   }

test/integration_test.dart

@@ -53,6 +53,12 @@ defineTests() {
           expect(collectingOut.trim(),
               stringContainsInOrder(['4 files analyzed, 0 issues found, in']));
         });
+        test('default', () {
+          dartlint.main(['test/_data/p2']);
+          expect(exitCode, 1);
+          expect(collectingOut.trim(),
+              stringContainsInOrder(['4 files analyzed, 3 issues found, in']));
+        });
       });
     });
     group('p3', () {