Allow package names to start with underscore.

sdk/lib/_internal/pub/lib/src/validator/name.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.validator.name;
 
 import 'dart:async';
 
 import 'package:path/path.dart' as path;
 
@@ skipping 52 matching lines @@
   void _checkName(String name, String description, {bool isPackage}) {
     // Packages names are more stringent than libraries.
     var messages = isPackage ? errors : warnings;
 
     if (name == "") {
       errors.add("$description may not be empty.");
     } else if (!new RegExp(r"^[a-zA-Z0-9_]*$").hasMatch(name)) {
       messages.add("$description may only contain letters, numbers, and "
           "underscores.\n"
           "Using a valid Dart identifier makes the name usable in Dart code.");
-    } else if (!new RegExp(r"^[a-zA-Z]").hasMatch(name)) {
-      messages.add("$description must begin with letter.\n"
+    } else if (!new RegExp(r"^[a-zA-Z_]").hasMatch(name)) {
+      messages.add("$description must begin with a letter or underscore.\n"
           "Using a valid Dart identifier makes the name usable in Dart code.");
     } else if (_RESERVED_WORDS.contains(name.toLowerCase())) {
       messages.add("$description may not be a reserved word in Dart.\n"
           "Using a valid Dart identifier makes the name usable in Dart code.");
     } else if (new RegExp(r"[A-Z]").hasMatch(name)) {
       warnings.add('$description should be lower-case. Maybe use '
           '"${_unCamelCase(name)}"?');
     }
   }
 
   String _unCamelCase(String source) {
     var builder = new StringBuffer();
     var lastMatchEnd = 0;
     for (var match in new RegExp(r"[a-z]([A-Z])").allMatches(source)) {
       builder
         ..write(source.substring(lastMatchEnd, match.start + 1))
         ..write("_")
         ..write(match.group(1).toLowerCase());
       lastMatchEnd = match.end;
     }
     builder.write(source.substring(lastMatchEnd));
     return builder.toString().toLowerCase();
   }
 }