Handle incomplete rule options gracefully.

pkg/analysis_server/test/services/linter/linter_test.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.
 
 library test.services.linter;
 
 import 'package:analysis_server/src/services/linter/linter.dart';
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/source/analysis_options_provider.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart';
 
 import '../../utils.dart';
 
 main() {
   initializeTestEnvironment();
   defineReflectiveTests(LinterRuleOptionsValidatorTest);
 }
 
 @reflectiveTest
 class LinterRuleOptionsValidatorTest {
-  final LinterRuleOptionsValidator validator= new LinterRuleOptionsValidator();
+  final LinterRuleOptionsValidator validator = new LinterRuleOptionsValidator();
   final AnalysisOptionsProvider optionsProvider = new AnalysisOptionsProvider();
 
   RecordingErrorListener recorder;
   ErrorReporter reporter;
 
   List<AnalysisError> get errors => recorder.errors;
 
   setUp() {
     recorder = new RecordingErrorListener();
     reporter = new ErrorReporter(recorder, new _TestSource());
@@ skipping 11 matching lines @@
 
   test_linter_no_rules() {
     validate(
         '''
 linter:
   rules:
     ''',
         []);
   }
 
+  test_linter_null_rule() {
+    validate(
+        '''
+linter:
+  rules:
+    -
+
+    ''',
+        []);
+  }
+
   test_linter_undefined_rule() {
     validate(
         '''
 linter:
   rules:
     - undefined
     ''',
         [UNDEFINED_LINT_WARNING]);
   }
 
   validate(String source, List<AnalysisOptionsErrorCode> expected) {
     var options = optionsProvider.getOptionsFromString(source);
     validator.validate(reporter, options);
     expect(errors.map((AnalysisError e) => e.errorCode),
         unorderedEquals(expected));
   }
 }
 
 class _TestSource implements Source {
   @override
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }