report errors in included options files

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 808c8353301b80cd4f71b208f34dd16ef53fc598..90cf8a123bb284ed942cd722844ce81f153db382 100644
--- a/pkg/analyzer/lib/source/analysis_options_provider.dart
+++ b/pkg/analyzer/lib/source/analysis_options_provider.dart
@@ -10,6 +10,7 @@ import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/source/source_resource.dart';
+import 'package:analyzer/src/task/options.dart';
 import 'package:analyzer/src/util/yaml.dart';
 import 'package:source_span/source_span.dart';
 import 'package:yaml/yaml.dart';
@@ -25,6 +26,8 @@ class AnalysisOptionsProvider {
   /// Provide the options found in either
   /// [root]/[AnalysisEngine.ANALYSIS_OPTIONS_FILE] or
   /// [root]/[AnalysisEngine.ANALYSIS_OPTIONS_YAML_FILE].
+  /// Recursively merge options referenced by an include directive
+  /// and remove the include directive from the resulting options map.
   /// Return an empty options map if the file does not exist.
   Map<String, YamlNode> getOptions(Folder root, {bool crawlUp: false}) {
     Resource resource;
@@ -42,17 +45,21 @@ class AnalysisOptionsProvider {
   }
 
   /// Provide the options found in [file].
+  /// Recursively merge options referenced by an include directive
+  /// and remove the include directive from the resulting options map.
   /// Return an empty options map if the file does not exist.
   Map<String, YamlNode> getOptionsFromFile(File file) {
     return getOptionsFromSource(new FileSource(file));
   }
 
   /// Provide the options found in [source].
+  /// Recursively merge options referenced by an include directive
+  /// and remove the include directive from the resulting options map.
   /// Return an empty options map if the file does not exist.
   Map<String, YamlNode> getOptionsFromSource(Source source) {
     Map<String, YamlNode> options =
         getOptionsFromString(_readAnalysisOptions(source));
-    YamlNode node = options.remove('include');
+    YamlNode node = options.remove(AnalyzerOptions.include);
     if (sourceFactory != null && node is YamlScalar) {
       var path = node.value;
       if (path is String) {
@@ -64,6 +71,8 @@ class AnalysisOptionsProvider {
   }
 
   /// Provide the options found in [optionsSource].
+  /// An include directive, if present, will be left as-is,
+  /// and the referenced options will NOT be merged into the result.
   /// Return an empty options map if the source is null.
   Map<String, YamlNode> getOptionsFromString(String optionsSource) {
     Map<String, YamlNode> options = <String, YamlNode>{};