Fix handling of empty .analysis_options files and test this case

pkg/analyzer/lib/source/analysis_options_provider.dart

Index: pkg/analyzer/lib/source/analysis_options_provider.dart
diff --git a/pkg/analyzer/lib/source/analysis_options_provider.dart b/pkg/analyzer/lib/source/analysis_options_provider.dart
index a3ac86bc4a8cb1e72c649fb3069c23f3f718abb1..fd20714a7a98479eca3bd86f6426388f4fe827d4 100644
--- a/pkg/analyzer/lib/source/analysis_options_provider.dart
+++ b/pkg/analyzer/lib/source/analysis_options_provider.dart
@@ -35,10 +35,6 @@ class AnalysisOptionsProvider {
       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) {