Fix handling of empty .analysis_options files and test this case

pkg/analyzer/test/source/analysis_options_provider_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.source.analysis_options_provider;
 
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/source/analysis_options_provider.dart';
 import 'package:unittest/unittest.dart';
 import 'package:yaml/yaml.dart';
@@ skipping 23 matching lines @@
       expect(ignore[0], equals('ignoreme.dart'));
       expect(ignore[1], equals('sdk_ext/**'));
     });
     test('test_doesnotexist', () {
       var optionsProvider = new AnalysisOptionsProvider();
       Map<String, YamlNode> options =
           optionsProvider.getOptions(resourceProvider.getFolder('/empty'));
       expect(options.length, equals(0));
     });
   });
+  group('AnalysisOptionsProviderEmpty', () {
+    setUp(() {
+      buildResourceProvider(true);
+    });
+    tearDown(() {
+      clearResourceProvider();
+    });
+    test('test_simple', () {
+      var optionsProvider = new AnalysisOptionsProvider();
+      Map<String, YamlNode> options =
+          optionsProvider.getOptions(resourceProvider.getFolder('/'));
+    });
+  });
 }
 
 MemoryResourceProvider resourceProvider;
 
-buildResourceProvider() {
+buildResourceProvider([bool emptyAnalysisOptions = false]) {
   resourceProvider = new MemoryResourceProvider();
   resourceProvider.newFolder('/empty');
   resourceProvider.newFolder('/tmp');
-  resourceProvider.newFile(
+  if (emptyAnalysisOptions) {
+    resourceProvider.newFile('/.analysis_options', r'''''');
+  } else {
+    resourceProvider.newFile(
       '/.analysis_options',
       r'''
 analyzer:
   ignore:
     - ignoreme.dart
     - 'sdk_ext/**'
 ''');
+  }
+
 }
 
 clearResourceProvider() {
   resourceProvider = null;
 }
 
 emptyResourceProvider() {
   resourceProvider = new MemoryResourceProvider();
 }