Attach source range information to parsed YAML nodes.

sdk/lib/_internal/pub/lib/src/barback.dart

 // Copyright (c) 2013, 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 pub.barback;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
@@ skipping 122 matching lines @@
             '"$key" field must be a string or list, but was "$field".');
       }
     }
 
     var includes = null;
     var excludes = null;
 
     if (configuration == null) {
       configuration = {};
     } else {
+      configuration = new Map.from(configuration);

[Bob Nystrom, 2014/06/03 00:33:14]

Document this.

Also, a nit, but can you add a blank line after this? I tend to prefer comment
lines have a blank line above them.

[nweiz, 2014/06/03 02:03:21]

Done.

       // Pull out the exclusions/inclusions.
       includes = parseField("\$include");
       excludes = parseField("\$exclude");
 
       // All other keys starting with "$" are unexpected.
       var reservedKeys = configuration.keys
           .where((key) => key is String && key.startsWith(r'$'))
           .map((key) => '"$key"');
 
       if (reservedKeys.isNotEmpty) {
@@ skipping 76 matching lines @@
   if (parts.length <= index + 1) {
     throw new FormatException(
         'Invalid URL path "${url.path}". Expected package name '
         'after "packages".');
   }
 
   var package = parts[index + 1];
   var assetPath = path.url.join("lib", path.url.joinAll(parts.skip(index + 2)));
   return new AssetId(package, assetPath);
 }