Fix handling of empty .analysis_options files and test this case

pkg/analyzer/lib/source/analysis_options_provider.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 source.analysis_options_provider;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:yaml/yaml.dart';
 
 /// Provide the options found in the `.analysis_options` file.
@@ skipping 17 matching lines @@
   }
 
   /// Provide the options found in [optionsSource].
   /// Return an empty options map if the source is null.
   Map<String, YamlNode> getOptionsFromString(String optionsSource) {
     var options = <String, YamlNode>{};
     if (optionsSource == null) {
       return options;
     }
     var doc = loadYaml(optionsSource);
-    if (doc is! YamlMap) {

[pquitslund, 2015/09/24 19:26:17]

I would prefer:

  if (doc != null && doc is! YamlMap)

It's still handy to identify poorly formatted options files.

[Cutch, 2015/09/24 19:37:22]

Done and I've added a test for the invalid yaml case.

-      throw new Exception(
-          'Bad options file format (expected map, got ${doc.runtimeType})');
-    }
     if (doc is YamlMap) {
       doc.forEach((k, v) {
         if (k is! String) {
           throw new Exception(
               'Bad options file format (expected String scope key, '
               'got ${k.runtimeType})');
         }
         options[k] = v;
       });
     }
     return options;
   }
 
   /// Read the contents of [file] as a string.
   /// Returns null if file does not exist.
   String _readAnalysisOptionsFile(File file) {
     try {
       return file.readAsStringSync();
     } on FileSystemException {
       // File can't be read.
       return null;
     }
   }
 }