Add support to pass the location of the analysis options file to plugins

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 analyzer.source.analysis_options_provider;
 
 import 'dart:core';
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 12 matching lines @@
 
   AnalysisOptionsProvider([this.sourceFactory]);
 
   /// 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;
+    return getOptionsFromFile(getOptionsFile(root, crawlUp: crawlUp));
+  }
+
+  /// Return the analysis options file from which options should be read, or
+  /// `null` if there is no analysis options file for code in the given [root].
+  ///
+  /// The given [root] directory will be searched first. If no file is found and
+  /// if [crawlUp] is `true`, then enclosing directories will be searched.
+  File getOptionsFile(Folder root, {bool crawlUp: false}) {
+    Resource resource = null;
     for (Folder folder = root; folder != null; folder = folder.parent) {
       resource = folder.getChild(AnalysisEngine.ANALYSIS_OPTIONS_FILE);
       if (resource.exists) {
         break;
       }
       resource = folder.getChild(AnalysisEngine.ANALYSIS_OPTIONS_YAML_FILE);
       if (resource.exists || !crawlUp) {
         break;
       }
     }
-    return getOptionsFromFile(resource);
+    return resource is File ? resource : null;
   }
 
   /// 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));
   }
 
@@ skipping 101 matching lines @@
 /// Thrown on options format exceptions.
 class OptionsFormatException implements Exception {
   final String message;
   final SourceSpan span;
   OptionsFormatException(this.message, [this.span]);
 
   @override
   String toString() =>
       'OptionsFormatException: ${message?.toString()}, ${span?.toString()}';
 }