Attach source range information to parsed YAML nodes.

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

 // Copyright (c) 2012, 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.pubspec;
 
 import 'package:yaml/yaml.dart';
 import 'package:path/path.dart' as path;
 
 import 'barback.dart';
@@ skipping 339 matching lines @@
 
       var versionConstraint = new VersionRange();
       if (spec == null) {
         description = name;
         sourceName = _sources.defaultSource.name;
       } else if (spec is String) {
         description = name;
         sourceName = _sources.defaultSource.name;
         versionConstraint = _parseVersionConstraint(spec, "$field.$name");
       } else if (spec is Map) {
+        // Don't write to the immutable YAML map.
+        spec = new Map.from(spec);
+
         if (spec.containsKey('version')) {
           versionConstraint = _parseVersionConstraint(spec.remove('version'),
               "$field.$name.version");
         }
 
         var sourceNames = spec.keys.toList();
         if (sourceNames.length > 1) {
           _error('"$field.$name" field may only have one source, but it had '
               '${toSentence(sourceNames)}.');
         }
@@ skipping 137 matching lines @@
 
     return "$str:\n$subMessage";
   }
 }
 
 /// Returns whether [uri] is a file URI.
 ///
 /// This is slightly more complicated than just checking if the scheme is
 /// 'file', since relative URIs also refer to the filesystem on the VM.
 bool _isFileUri(Uri uri) => uri.scheme == 'file' || uri.scheme == '';