Add an AnalysisOptionsProvider to analyzer

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.
+class AnalysisOptionsProvider {
+  /// The name of the analysis options source file.
+  static const String ANALYSIS_OPTIONS_NAME = '.analysis_options';
+
+  /// Provide the options found in the [ANALYSIS_OPTIONS_NAME] file located in
+  /// [folder]. Return an empty options map if the file does not exist.
+  Map<String, YamlNode> getOptions(Folder root) {
+    var options = <String, YamlNode>{};
+    var optionsSource = _readAnalysisOptionsFile(root);
+    if (optionsSource == null) {
+      return options;
+    }
+    var doc = loadYaml(optionsSource);
+    if (doc is! YamlMap) {
+      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 [root]/[ANALYSIS_OPTIONS_NAME] as a string.
+  /// Returns null if file does not exist.
+  String _readAnalysisOptionsFile(Folder root) {
+    var file = root.getChild(ANALYSIS_OPTIONS_NAME);
+    try {
+      return file.readAsStringSync();
+    } on FileSystemException {
+      // File can't be read.
+      return null;
+    }
+  }
+}