Map merging.

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:analyzer/src/generated/engine.dart';
+import 'package:analyzer/src/util/yaml.dart';
 import 'package:source_span/source_span.dart';
 import 'package:yaml/yaml.dart';
 
 /// Provide the options found in the `.analysis_options` file.
 class AnalysisOptionsProvider {
   /// Provide the options found in [root]/[ANALYSIS_OPTIONS_FILE].
   /// Return an empty options map if the file does not exist.
   Map<String, YamlNode> getOptions(Folder root) {
     var optionsSource = _readAnalysisOptionsFile(
         root.getChild(AnalysisEngine.ANALYSIS_OPTIONS_FILE));
@@ skipping 40 matching lines @@
               'Bad options file format (expected String scope key, '
               'got ${k.runtimeType})',
               k != null ? k.span : doc.span);
         }
         options[k] = v;
       });
     }
     return options;
   }
 
+  /// Merge the given options contents where the values in [defaults] may be
+  /// overridden by [overrides].
+  ///
+  /// Some notes about merge semantics:
+  ///
+  ///   * lists are merged (without duplicates).
+  ///   * lists of scalar values can be promoted to simple maps when merged with
+  ///     maps of strings to booleans (e.g., ['opt1', 'opt2'] becomes
+  ///     {'opt1': true, 'opt2': true}.
+  ///   * maps are merged recursively.
+  ///   * if map values cannot be merged, the overriding value is taken.
+  ///
+  Map<String, YamlNode> merge(
+          Map<String, YamlNode> defaults, Map<String, YamlNode> overrides) =>
+      new Merger().merge(defaults, overrides);
+
   /// 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;
     }
   }
 }
 
 /// 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()}';
 }