Allow a wider pub constraint on barback.

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;
 
 import 'utils.dart';
 import 'version.dart';
 
-/// The currently supported version of the Barback package that this version of
+/// The currently supported versions of the Barback package that this version of
 /// pub works with.
 ///
-/// Pub implicitly constrains barback to this version or later patch versions.
+/// Pub implicitly constrains barback to these versions.
 ///
 /// Barback is in a unique position. Pub imports it, so a copy of Barback is
 /// physically included in the SDK. Packages also depend on Barback (from
 /// pub.dartlang.org) when they implement their own transformers. Pub's plug-in
 /// API dynamically loads transformers into their own isolate.
 ///
 /// This includes a string literal of Dart code ([_TRANSFORMER_ISOLATE] in
 /// load_transformers.dart). That code imports "package:barback/barback.dart".
 /// This string is included in the SDK, but that import is resolved using the
 /// application’s version of Barback. That means it must tightly control which
 /// version of Barback the application is using so that it's one that pub
 /// supports.
 ///
 /// Whenever a new non-patch version of barback is published, this *must* be
 /// incremented to synchronize with that.
-final supportedVersion = new Version(0, 12, 0);
+final supportedVersions = new VersionConstraint.parse(">=0.11.0 <0.13.0");
 
 /// A list of the names of all built-in transformers that pub exposes.
 const _BUILT_IN_TRANSFORMERS = const ['\$dart2js'];
 
 /// An identifier for a transformer and the configuration that will be passed to
 /// it.
 ///
 /// It's possible that the library identified by [this] defines multiple
 /// transformers. If so, [configuration] will be passed to all of them.
 class TransformerId {
@@ skipping 187 matching lines @@
 
   if (id.package != entrypoint) {
     throw new FormatException(
         'Can only access "lib" and "asset" directories of non-entrypoint '
     'packages.');
   }
 
   // Allow any path in the entrypoint package.
   return path.url.join("/", path.url.joinAll(parts));
 }